Molecules containing heterocyclic substructures continue to be attractive targetsfor synthesis since they often exhibit diverse and important biological properties 96CHC1. The efficiency with which heterocycles can be constructed is importantnot only because it affects the production costs for the desired material, but also theenvironmental impact associated with waste disposal, conservation of source ma-terials like petroleum stocks, and energy consumption.
The rate of increase in mo-lecular intricacy as one progresses from simple starting materials to the final productcan also serve as a measure of efficiency 01T On one end of the continuum, asingle synthetic step could convert an inexpensive material into a highly complexheterocyclic product.
On the other end lies a linear series of transformations, whereina single atom or group is added in each step to build complexity. As a prerequisitefor an ideally proceeding one-pot sequential transformation, the reactivitypattern of all participating components has to be such, that each building block isinvolved in the reaction only when required to do so.
The reality of chemicalsynthesis is somewhere between these extremes, with the one-step process held asthe ideal. Because of the rate at which they increase molecular intricacy, cascadereactions have received considerable attention from the synthetic organic commu-nity.
The development of sequences that combine transformations of differingfundamental mechanism broadens the scope of such procedures in syntheticchemistry. This review contains a representative sampling from the last 15 years on the kindsof reactions that have been sequenced into cascades to produce heterocyclic mol-ecules. The fact that multiple reactions give rise to a cascade sequence makes thecategorization of these processes difficult.
The structure we have imposed, therefore,is somewhat arbitrary but is loosely based on what, in our judgment, is the keyreaction in the cascade sequence.
This mini-review is not intended to be a critical orcomprehensive coverage, but rather provides an overview of the field and thus somecascade processes are covered in more detail than others. Many different examples of cascade processes that employ 1,3-dipoles as reactiveintermediates have been described in the literature. The transition metal-catalyzeddecomposition of diazoimides results in the formation of isomunchnone dipoles, aclass of mesoionic betaines 03SA, 99JOC, 00JOC , which are known toreadily undergo 1,3-dipolar cycloaddition chemistry.
Ibata and Hamaguchi were thefirst to report that diazoimide 1 formed isomunchnone 2 on heating in the presence ofCu2 acac 2 74TL , and that this reactive dipole could be trapped with variousdipolarophiles such as 3 to give the oxabicyclic product 4 Scheme 1 75CL By tethering thedipolarophile moiety within the diazoimide, complex polycyclic frameworks can beformed in a single step 88TL Over a period of years, Padwa and coworkers demonstrated that thiscascade sequence is quite general.
Diazoimides with the general structure 7 n 1, 2,3; m 1, 2 were readily converted to the corresponding polycyclic system 8 92TL Compounds of type 6 and 8 contain an N,O-acetal functional group and have beenused as precursors to N-acyliminium ions. This method was exploited for the syn-thesis of B-ring homologs of the erythrinane family of alkaloids 97JOC In thesestudies, diazoimi Post on Dec views.
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DOI: Download Citation: Org. Modern advances in heterocyclic chemistry in drug discovery A. Taylor, R. Its current importance can be measured by the fact that, of the nearly references, approximately half date from the last 10 years. Preparation of aminoisoxazoles and their utility in the synthesis of condensed systems are also covered. In the final chapter, isothiazolium salts and their use in synthesis are reviewed.
Many condensed S,N-heterocyclic systems are described in this, the first review dedicated to this topic. Alan Katritzky was educated at Oxford and has held faculty positions at Cambridge and East Anglia before he migrated in to the University of Florida, where he was Kenan Professor and Director for the Institute for Heterocyclic Compounds.
During his career he has trained more than graduate students and post-docs, and lectured and consulted world-wide. Honors include 14 honorary doctorates from 11 countries and membership of foreign membership of the National Academies of Britain, Catalonia, India, Poland, Russia and Slovenia.
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