Towards the total synthesis of chondrochloren A: synthesis of the (Z)-enamide fragment

The stereoselective synthesis of the (Z)-enamide fragment of chondrochloren (1) is described. A Buchwald-type coupling between amide 3 and (Z)-bromide 4 was used to generate the required fragment. The employed amide 3 comprising three chiral centers was obtained through a seven-step sequence starting from ᴅ-ribonic acid-1,4-lactone. The (Z)-vinyl bromide 4 is accessible in four steps from 4-hydroxybenzaldehyde. The pivotal cross coupling between both fragments was achieved after extensive experimentation with copper(I) iodide, K2CO3 and N,N′-dimethylethane-1,2-diamine.

Synthesis of a C1–C12 Fragment of Gulmirecin B

The synthesis of a C1–C14 fragment of the macrolide antibiotic gulmirecin B through formation of the C7–C8 bond by addition of a vinyllithium intermediate to a C1–C7 aldehyde was investigated. This crucial coupling was successful with a vinyllithium reagent corresponding to a C8–C12 fragment. The C8–C12 vinyl bromide was prepared from l-malic acid. The C1–C7 aldehyde building block was synthesized from hex-5-enoic acid by using an Evans alkylation, a cross-metathesis, and an asymmetric dihydroxylation as key steps.