Further Studies on the [1,2]-Wittig Rearrangement of 2-(2-Benzyloxy)aryloxazolines

Abstract

The behaviour of 14 ortho-functionalised 2-aryloxazolines (11 of them prepared and characterised for the first time) with butyllithium has been examined. Significant limitations to the Wittig rearrangement of such systems are revealed. In terms of asymmetric Wittig rearrangement, good diastereoselectivity is obtained with a valine-derived 4-isopropyl oxazoline, but this is compromised by racemisation upon hydrolysis. More encouraging selectivity is achieved in the Wittig rearrangement of an acyclic phenylalanine-derived ortho-benzyloxy benzamide.

Synthesis and Wittig Rearrangement of 3- and 4-Benzyloxyphenylphosphonamidates

“…In the meantime, we developed the N-butylcarboxamide, CONHBu as a more effective and general activating group, facilitating Wittig rearrangement of ortho-, meta-, or para-disposed benzylic ethers 5 to give diarylmethanols 6 [6]. Limited success in using a chiral secondary amide group to direct asymmetric Wittig rearrangement was also reported [7].…”

A tribute to Professor Aitken

“…63 This method has since been extended to the phosphonamide directing group giving 71, 64 and further studies on oxazoline-based systems have also been reported. 65 Arising from a fundamental study on FVP of a-functionalised carboxylic acid derivatives, dioxolanones derived from lactic or mandelic acid were found to be effective chiral acyl anion equivalents. This is exemplified by Michael addition of dioxolanone 72 to ethyl crotonate followed by FVP to give product 73 in 86% e.e.…”

Synthesis and Butyllithium-Induced Cyclisation of 2-Benzyloxyphenylphosphonamidates Giving 2,3-Dihydrobenzo[d][1,3]oxaphospholes

“…More recently, we discovered the N-butylcarboxamide CONHBu as a more efficient and general activating group, allowing the Wittig rearrangement of ortho-, meta-, or paraoriented benzylic ethers 5 to afford diarylmethanols 6 [6]. A limited degree of success in using a chiral secondary amide group to bring about asymmetric Wittig rearrangement was also described [7]. As far as we are aware, there is only a single report of an enantioselective [1,2]-Wittig rearrangement, and this uses an external chiral bis(oxazoline) ligand [8].…”

“…In an earlier paper, we described the synthesis of aryl benzyl ethers bearing the phosphonamidate group EtO-P(=O)-NHBu on the aryl ring, either para- (7) or meta- (9) to a benzylic ether, and their successful Wittig rearrangement to afford the corresponding phosphonamidate-functionalised diarylmethanols 8 and 10, respectively (Scheme 2) [9]. In this paper, we describe the synthesis of a series of the isomeric aryl benzyl ethers 11 bearing an ortho-phosphonamidate group and their reaction with butyllithium, which leads not to Wittig rearrangement but rather to cyclisation, giving 2,3-dihydrobenzo[d] [1,3]oxaphospholes 12.…”