Selective uni- and bidirectional homologation of diborylmethane

Abstract

Diborylmethane can be homologated uni- and bidirectionally by using enantiomerically pure lithium-stabilized carbenoids to give 1,2- and 1,3-bis(boronic esters).

Synthesis of the C18-C27 fragment of georatusin

“…Our synthesis started with the gram-scale preparation of known C2-symmetric 1,3-bis(boronic ester) 3 18,19 employing our protocol. 19 Starting from our key building block 3 the precursor of the other two fragments is readily available using a three step sequence consisting of our mono-Zweifel olefination protocol, 19 Mattesonhomologation 20 and oxidation (Scheme 2).…”

“…For the synthesis of the new middle fragment 13, (−)-pseudoephedrine (16) was propionated. 23,24 Then, the (R)-Roche ester (18) was transformed into iodide 19 in a known three step sequence. 25 A literature known sequence consisting of Myers alkylation and reductive removal of the auxiliary gave us the corresponding alcohol 26 which was then converted into middle fragment 13 using a Mitsunobu reaction 22 (Scheme 5).…”

Vinylidene Homologation of Boronic Esters and its Application to the Synthesis of the Proposed Structure of Machillene

“…28 8), allowing us to eliminate six of the possible eight diastereoisomers.Based on chemical shift differences between the methylene protons, Breit assigned the methyl substituents of 10 a as having an anti-relationship (Dd = 0ppm observed, Dd > 0.5 ppm for the syn isomer). [32] Compound 10 b was therefore identified as the most likely structure of the natural product and we set out to confirm its structure through synthesis.O ur retrosynthetic analysis began with sequential epoxidation reactions from tetraene 11.W ee nvisaged the carbon skeleton would be constructed through abidirectional approach from methylene diboron (14), comprised of homologations with lithiated TIB ester 13 [33] and lithiated epoxysilane 4a,f ollowed by aP dcatalyzed allylic cross-coupling reaction. If successful, the route would give access to machillene in just 5s teps from commercially available diboron 14,and without the need for protecting groups.…”

A Complementary Toolbox of Iterative Methods for the Stereo­selective Synthesis of Heteroatom-Rich Motives from C1-Building Blocks

“…These insert into the C-B bond to stereospecifically deliver homologation products like 70a. 63 Sterically unhindered boronates react significantly faster with carbenoids of type 66 than others, 64 but homologation of primary, secondary, and tertiary (vide infra) substrates is possible.…”