ISSN:
0947-3440
Keywords:
Cycloadditions
;
C=S Dienophile
;
N-Arylmonothiomaleimides
;
Chemo-, regio-, and stereoselectivity
;
AM1 calculation
;
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Both the C=S and C=C functionalities of N-arylmonothiomaleimides readily undergo a [4 + 2] cycloaddition reaction at ambient temperatures with both electron-donating and -withdrawing dienes. The C=S groups of N-arylmonothiomaleimides are generally more reactive than the C=C groups, especially toward dienes bearing conjugation and electron-attracting groups, and provide mixtures of ortho,endo products 3 and ortho,exo products 4 with complete regioselectivity (100% ortho) and with high stereoselectivity (more than 86% endo). The C=C groups, on the other hand, display a high stereoselectivity (100% endo) but a low regioselectivity, providing ortho,endo adducts 5 and meta,endo adducts 6 in ratios ranging from 1:1 to 6:1. Compared with N-phenylmonothiomaleimide (1). N-p-nitrophenylmonothiomaleimide (11a) is more reactive and displays a higher C=S vs. C=C chemoselectivity in favor of C=S, while N-p-anisyl (11b) and N-benzyl (11c) derivatives display quite the reverse reactivity and selectivity. N-Phenylmonothiocitraconimide (11d) serves as a C=S specific dienophile and reacts with dienes to furnish ortho,endo adducts 12 with excellent regio- and stereoselectivities (100% ortho,endo). Both hard (BF3 · OEt2, TiCl4) and soft (CuBr · SMe2) Lewis acids enhance the reaction rate enormously and increase chemoselectivity in favor of C=S. The chemo- (C=S/C=C), regio- (ortho/meta) and stereochemical outcomes (endo/exo) are rationalized by transition structure analysis based on AM1 calculation.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jlac.199719970520
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