Highly enantioenriched 4-hydroxy-1-alkenyl N,N-diisopropylcarbamates, easily available by asymmetric homoaldol reaction, cyclize by treatment with sodium hydride to form (1r,2t,3t)-configured 1-acylcyclopropanes with high diastereoselectivity. The decisive steps are the migration of the N,N-diisopropylcarbamoyl group onto the alkoxide oxygen atom, followed by an intramolecular homoallylic substitution
Convenient Synthesis of Enantiomerically Enriched (Oxybutenyl)stannanes and Their Lewis Acid Catalyzed Homoaldol Reactions
作者:Holger Paulsen、Claudia Graeve、Dieter Hoppe
DOI:10.1055/s-1996-4172
日期:1996.1
(E)-But-2-enyl N,N-diisopropylcarbamate10 (5), after deprotonation to its lithium/(-)-sparteine complex 6, yields with trialkyltin chlorides mixtures of the 3-oxy- and 1-oxy-substituted, enantiomerically enriched allylstannanes (S)-8 and (R)-9. (R)-8 is obtained with high regioselectivity and enantiomeric purity (95% ee) via a delithiotitanation (inversion) and detitanostannylation (anti-SE’)sequence. The titanium tetrachloride mediated condensation of (S)- or (R)-8 with aldehydes or ketones proceeds via titanodestannylation to yield (Z)-anti-homoaldol products 11 with complete chirality transfer.
Synthesis of an Enantioenriched α-Carbamoyloxy-crotylboronate and Its Homoaldol Reaction with Aldehydes
作者:Dieter Hoppe、Edith Beckmann
DOI:10.1055/s-2004-834921
日期:——
Enantioenriched α-carbamoyloxy-crotylboronate 5 was prepared via the corresponding γ-stannylated carbamate. This boronate was then subjected to homoaldol reactions with aromatic and aliphatic aldehydes furnishing (Z)-anti-homoallylic alcohols in high diastereoselectivity and with complete chirality transfer.