Heterocycles in the Service of Humankind

Alleviating the scourge of biting insects; Bioconjugates for new directions in pharmaceutical research; Sharing the benefits of research

clothing.The repellent most often used is DEET but a major drawback is that DEET can lead to adverse side effects in some individuals, especially from its systemic uptake via dermal absorption.
New, improved long lasting repellents that are safe and efficacious against a wide range of insects are needed.Thus, there is a long-standing interest in the design of compounds that will be effective repellents against mosquitoes, sand flies, stable flies, black flies, tsetse flies, biting gnats and tabanids, all of which spread human and animal diseases.
Although the design of new insect repellents by computer-aided molecular modeling was discussed in a recent book, there have been few previous attempts to correlate the repellency of compounds towards mosquitoes with their chemical structure. 2Suryanarayana et al. 1  With the same data-set, and using descriptors derived solely from the chemical structures of the repellants we found an improved model with an R 2 of 0.79.This encouraged us to go further.Over the last few years we have been collaborating with the U.S. Department of Agriculture at the University of Florida, which over the past 60 years have recorded tests covering a wide range of insect species and some 30,000 different compounds.Properties such as protection time, effective dose and lethal dose towards a large numbers of insects including various species of mosquitoes, houseflies and other pests were amassed.We were given access to these records and in agreement with the Department of Agriculture embarked on a program to try and extract from them some information that would allow us to understand the relationship between the biological activity of compounds and their chemical structures.This lecture concerns the work that we have carried out with N-acylpiperidines, and which is described in detail in Proc.Nat.Acad.Sci.USA, 2008.
The USDA records included more than 150 different N-acylpiperidines which had been tested for their effectiveness as mosquito repellants.These compounds were classified according to their effectiveness into five classes, 1 being the least active and class 5 being the most active.The most active compounds retained effectiveness for 21 days.By building a neural network model, we were able to correlate protection times from the old USDA data with the chemical structures as is shown in Figure 1.We re-synthesized 11 of the N-acylpiperidines that were previously tested in order to anchor the envisaged biological testing into the existing data.Piperidines were synthesized utilizing acylbenzotriazoles prepared from carboxylic acids by treatment with thionyl chloride and benzotriazole.In addition to the 11 previously tested N-acylpiperidines, we synthesized 23 novel N-acylpiperidines as potential repellants.These 23 compounds were selected by first using our neural network model to predict the likely activity of many hundreds of compounds.Most of the 23 compounds selected were expected to be highly active but a few were also chosen that were predicted to have lower activity.
Biological testing was carried out for all 23+11=34 compounds synthesized, together with DEET.Bioassays were conducted by covering the hand of a volunteer with a soft-embossed long cuff poly glove and powder-free latex glove.To cover the arm a stocking is pulled over.A sleeve with an opening (3 cm x 8 cm) was fastened around the arm.Each cloth patch assembly was affixed over the open window with masking tape to hold it in place on the sleeve.The arm is then inserted into the cage of mosquitoes and held stationary for 1 minute.The number of feeding mosquitoes was counted prior to removal with a quick, brisk shake of the arm.Feeding mosquitoes that remained in the window were considered to have been biting.
The failure threshold for repellency for these experiments was set at 1% biting (5 bites) confirmed by achievement of two consecutive days of 5 or more bites.The results are shown in Table 1 and graphically in Figure 2. Gratifyingly, several of the compounds prepared showed considerably improved protection times compared to DEET.The structures of the most effective compounds and their protection times are compared with DEET in Figure 3.These compounds are now being subjected to toxicity and other tests to see if they can be applied.We have particularly not asked for any patent protection for any of this work in order to make the results available to the whole world as quickly as possible.The pharmaceutical industry is in crisis.It is becoming increasingly difficult to find new major drugs.Many of the time-honored strategies for drug research seem to be failing.One direction that is opening up is that of bioconjugates.Bioconjugates can be defined as compounds in which at least two fundamentally different types of organic structure are linked together.Table 2 shows the possibilities considering two out of six compound classes to be linked to each other or to another class.Some of the different types of bioconjugates displayed in Table 2 are very well known, but others have hardly been studied.

Table 2. Bioconjugates for new directions in pharma research
An easy and often convenient way of linking two structural units together in organic chemistry is acylation.The classical way to affect acylation uses acid chlorides, but there are disadvantages.Recently 3 we introduced N-acylbenzotriazoles as substitutes for acid chlorides.Several advantages are associated with these reagents: 1. Preparation: rapid one-pot procedures with mild reaction conditions (THF; 20 o C; 2 hr) BtH + SOCl 2 → [BtSOBt] + RCO 2 H → RCOBt; RCO 2 Na + BtTs → RCOBt +TsONa.2. Isolation: easily in crystalline form; high yields and purity without chromatography.3. Stability: can be weighed out and handled in air, and stored at 20°C for many weeks.
4. Insensitive to water and thus can be used in partly aqueous solution: this allows peptidecoupling using amino acids with free carboxyl groups.5. Protection not usually required for aliphatic or aromatic -OH, heterocyclic -NH, -SH, or -CONH 2 .6.They are more reactive and more crystalline than acyl imidazoles.7. The Bt group is readily replaced by N-, S-, O-, and C-nucleophiles.
Acylbenzotriazoles can be applied to classical problems such as the synthesis of "difficult" peptide sequences, one of which is shown in Figure 4.    Another application 5 is in the preparation of α-aminoacylamino-substituted heterocycles which show many diverse biological activities.Figure 5 shows some examples of this, comparing the new methodology with the old, and resulting in the preparation of a diverse range of such compounds -additional examples of which are shown in Figure 6.
We have linked amino acids with terpenes through hydroxyl groups, as summarized in Figure 7. 6 The data in Figure 7 also demonstrate the fact that chirality is preserved in the preparation and reactions of acylbenzotriazoles.Thus DL-Phe-O-citronellol shows retention times corresponding to each of the two diastereoisomers, whereas the analogous L-phenylalanine product shows only one peak.Similarly amino-acid residues can be linked to steroids: Figure 8 shows some bioconjugates derived from cholesterol linked to a variety of amino acids.Tagging has become extremely important for biomolecules (Figure 9).Azo-dye-labeled O-aminoacyl terpenes, -sugars, and -steroids 8 have been prepared by the new technique, as illustrated in Figure 10.Fluorescent markers are often even more effective than dye markers.We have shown that coumarin units can be utilized in combination with lysine chemistry to tag a variety of amino acids and peptides 9 (Figure 11).
Incorporation of a sugar unit confers solubility on such compounds and some examples of this technique 10 are shown in Figure 12.
A series of dye-labeled nucleosides 11 is shown in Figure 13.Similar techniques have been applied to dye-label amino acids and dipeptide amino alcohols, 12 as shown in Figure 14.

Attempts to help Chemistry in Developing Countries by Innovations in the Publishing and Dissemination of Organic Chemistry Research
The problems faced by libraries in meeting the high subscriptions required by commercial publishers are well known.The subscriptions set by learned societies for access to their publications are also high.This has given rise to the so-called "open access journals" where there is no charge to the reader, but page charges are levied on authors.In these circumstances it was decided eight years ago to launch a new journal with a very different philosophy: it would be free to authors, with no page charges or other fees, and also free to readers with no access or downloading charges.Thus Arkivoc was designed for universal on-demand distribution at no cost to authors or subscribers (see Figure 15).Many of the standard publishing procedures are used in Arkivoc, as detailed in Figure 16, but a major difference is that the "Control Board" which runs Arkivoc is unpaid.The composition of the Control Board as of spring 2008 is shown in Figure 17.We also have a very large Editorial Board of Referees.This has recently been extended significantly, and we now have close to 1,000 members.One difference from most editorial boards of referees is that we have about half our members from outside Western Europe, North America or Japan (see Figures 18 and 19) for representative lists.
During the eight years of existence, Arkivoc has progressed significantly in the number of manuscripts received and published.The number of visitors to its website has also increased dramatically; we now have about 100,000 visitors per month who make more than a million hits per month on the website.An important part of the philosophy of ARKIVOC is to honor chemists from around the world.Distinguished scientists, including chemists, have long been honored in their own countries and a small number, mainly from a few Western developed countries, are honored internationally.Arkivoc has chosen to recognize chemists from around the world and, so far we have produced 85 commemorative issues (Figure 20).Our impact factor has risen steadily and has now overtaken that of several other journals (Figure 21); while it is still well below that of the best journals in organic chemistry, we hope that the increase in our impact factor will continue.
We have also tried to improve contacts between chemists worldwide by running an annual heterocyclic and synthetic conference at the University of Florida.At this conference we offer a dozen plenary lectures from some of the world's most distinguished chemists 22) together with short courses on various aspects of organic chemistry, 20-30 invited lectures, some 60 posters, and a full social program.Figure 23     We hope that all chemists will help our efforts to extend the hand of friendship to organic chemists all around the world.We now have a rather high rejection rate for Arkivoc, but if you submit a manuscript we will try to provide you with constructive criticism that may be helpful even if it is rejected.We hope that you will access ARKIVOC and, when relevant, cite ARKIVOC; the subject index will help you find suitable papers.
We also hope that you will consider coming to the Flohet conference; it gives excellent value and all profits go to support ARKIVOC.We only regret that we cannot reduce fees or give any awards because the whole operation of ARKIVOC and Flohet occurs without major support and is only made possible by the unstinting efforts of a large number of community-minded chemists.
used a small set of 31 repellants to propose the model (eqn 1) with the rather low R 2 of 0.304: where PT = Protection Time; log P = lipophilicity; Vp = Vapor pressure; ML = Molecular Length.PT = a logP + b logVp + c log ML + d (1)

Figure 1 .
Figure 1.Correlation of protection times (old USDA data) with chemical structures by ANN.

Figure 2 .
Figure 2. Correlation of protection time with chemical structure for the newly synthesized Nacylpiperidines.

Figure 4 . 4 ISSN
Figure 4. Microwave-assisted solid phase synthesis of a short difficult pentapeptide.4

Figure 16 .
Figure 16.Submitting, refereeing and editing of manuscripts in ARKIVOC.

Figure 19 .
Figure19.ARKIVOC Editorial Board of Referees: members from outside W. Europe, N. America, Japan.
shows the program for the 2008 conference.