respective amino and hydroxy acids. A new synthetic route to unit A allows the selective generation of all four stereogenic centres in a short, efficient and reliable synthesis and contributes to an easier and faster synthesis of cryptophycins. The first two stereogenic centres are introduced by a catalytic asymmetricdihydroxylation, whereas the remaining two stereogenic centres are introduced with
[reaction: see text] Two short synthetic approaches toward cryptophycin unit A comprise a catalytic asymmetric dihydroxylation as the sole source of chirality, while all further stereogenic centers are introduced under substrate control. The key step of the first route is a vinylogous Mukaiyama aldol addition, which introduces the alpha,beta-unsaturated ester moiety with defined configuration at the
regio-, diastereo-, and enantiocontrolled access to alpha,beta-unsaturated delta-lactones is described, based on the reaction of a silyl dienolate and an aldehyde in the presence of 10 % of Carreira's catalyst. The scope and limitations of this reaction, as well as mechanisticinsights concerning the reactivity of an allyl copper species, are discussed.
O-Silylated dienolates in organic synthesis: γ-Selective alkylation of unsaturated carbonyl compounds by 1,5-dithienium fluoroborate.
作者:Ian Paterson、Lee G. Price
DOI:10.1016/s0040-4039(01)90565-9
日期:1981.1
The alkylation of O-silylated dienolates of unsaturated aldehydes, ketones, and esters with 1,3-dithienium fluoroborate shows useful γ-selectivity. The γ-products are selectively protected 1,5-dicarbonyl compounds.
A tunable and highly regio- and diastereoselective addition of acyclic silyl dienolates 2 to several alpha-fluoroalkyl sulfinylimines 1 was developed. By appropriate choice of the Lewis acid catalyst, two new chiral alpha-fluoroalkyl amines 3 and 4 were obtained in good yields and excellent diastereoselectivities (up to >99 : 1 dr), respectively.