Asymmetric [4 + 3] Cycloadditions between Benzofuranyldiazoacetates and Dienes: Formal Synthesis of (+)-Frondosin B

Catalyst-controlled formal [4 + 3] cycloaddition applied to the total synthesis of (+)-barekoxide and (-)-barekol

The tandem cyclopropanation/Cope rearrangement between bicyclic dienes and siloxyvinyldiazoacetate, catalyzed by the dirhodium catalyst Rh(2)(R-PTAD)(4), effectively accomplishes enantiodivergent [4 + 3] cycloadditions. The reaction proceeds by a cyclopropanation followed by a Cope rearrangement of the resulting divinylcyclopropane. This methodology was applied to the synthesis of (+)-barekoxide (1) and (-)-barekol (2).

Gold-catalyzed Intermolecular [4C + 3C] Cycloaddition Reactions

In the presence of the N-heterocyclic carbene gold catalyst (NHC-AuIPr, 7), propargyl esters 1a-f and 13 undergo a [4C + 3C] cycloaddition reaction with cyclopentadiene and furan under mild conditions. The evidence suggests the formation of the seven-membered ring occurs by a direct cycloaddition process, rather than a stepwise cyclopropanation/Cope rearrangement sequence.

The organocatalytic three-step total synthesis of (+)-frondosin B

The frondosins are a family of marine sesquiterpenes isolated from the sponge Dysidea frondosa that exhibit biological activities ranging from anti-inflammatory properties to potential application in anticancer and HIV therapy. Herein, a concise enantioselective total synthesis of (+)-frondosin B is described which requires a total of three chemical steps. The enantioselective conjugate addition of a benzofuran-derived boronic acid to crotonaldehyde in the presence of an imidazolidinone organocatalyst builds the critical stereogenic center of frondosin B in the first operation, while the remaining two ring systems of this natural product are installed in the two subsequent steps. A combination of X-ray crystallographic data, deuterium labeling, and chemical correlation studies provides further evidence as to the correct absolute stereochemical assignment of (+)-frondosin B.

Application of donor/acceptor-carbenoids to the synthesis of natural products

The metal catalyzed reactions of diazo compounds have been broadly used in organic synthesis. The resulting metal-carbenoid intermediates are capable of undergoing a range of unconventional reactions, and due to their high energy, they are ideal for initiating cascade sequences leading to the rapid generation of structural complexity. This tutorial review will give an overview of the most versatile reactions of donor/acceptor carbenoids, an exciting class of intermediates capable of highly selective reactions. This will include cyclopropanation, [4 + 3] cycloaddition, and C-H functionalization methodologies. The application of this chemistry to the synthesis of a range of natural products will be described.

Asymmetric synthesis of seven-membered carbocyclic rings via a sequential oxyanionic 5-exo-dig cyclization/claisen rearrangement process. Total synthesis of (-)-frondosin B

Appropriately substituted nonracemic allyl alcohols, readily prepared from the corresponding enones by application of the CBS methodology, were converted to optically active cycloheptenone derivatives with almost complete transfer of chirality via an efficient "one-pot", cycloisomerization/Claisen rearrangement process. This methodology was directly applied to the expedient total synthesis of (-)-frondosin B.

Enantioselective reactions of donor/acceptor carbenoids derived from alpha-aryl-alpha-diazoketones

The reaction of a variety of alpha-aryl-alpha-diazo ketones with activated olefins, catalyzed by the adamantyl glycine-derived dirhodium complex Rh(2)(S-PTAD)(4), generates cyclopropyl ketones with high diastereoselectivity (up to >95:5 dr) and enantioselectivity (up to 98% ee). Intermolecular C-H functionalization of 1,4-cyclohexadiene by means of carbenoid-induced C-H insertion was also possible with this type of carbenoid.