Thiocarbonyl ylides and electrophilic azo compounds; Sterically hindered cyclic hydrazodicarboxylic esters

The cycloadduct 5, prepared from adamantanethione S-methylide with dimethyl azodicarboxylate, exists in solution in two conformations P and Q (70:30 i n CDCl3), which are separated by a barrier of Delta G not equal 18.3 +/- 0. 6 kcal mol(-1); according to NMR spectra, both forms are chiral. Steric hin drance to 3'N - CO2CH3 rotation by the adamantane system generates C=O outs ide and C=O inside conformations. The barrier to enantiomerization (AB --> A(2) for 5'-H-2) is higher (> 20.1 kcal mol(-1)). Three combinations of rat e processes are discussed. Except for the 5'-methyl compound 19a, the chira l cycloadducts of four other thiocarbonyl ylides to dimethyl azodicarboxyla te do not show diastereoisomeric conformations in their NMR spectra at ambi ent temperature. (C) 1999 Elsevier Science Ltd. All rights reserved.