Enantioselective Catalytic Cyclopropanation-Rearrangement Approach to Chiral Spiroketals

Recent advances in asymmetric synthesis via cyclopropanol intermediates

Cyclopropanols have attracted significant attention in organic synthesis as versatile three-carbon synthons, as this readily available class of donor-activated cyclopropanes undergoes miscellaneous transformations, either via ring-opening or with retention of the cyclopropane ring. This review summarizes stereoselective and stereoretentive transformations suitable for asymmetric synthesis. The utility of cyclopropanols is discussed for two main strategies: (i) substrate-controlled transformations using enantiomerically enriched cyclopropanol intermediates through a traditional approach, and (ii) the use of nonchiral or racemic cyclopropanols, where asymmetric induction is achieved through a chiral catalyst, representing a direction that has recently emerged.

Intramolecular Oxyalkylation of Unactivated Alkenes

The synthesis of cyclopropanes by the cyclization of allylic diazoesters is well-known. In prior studies toward the sesquiterpenoid euonyminol, we attempted to carry out an intramolecular cyclopropanation of an allylic diazoester containing an electronically-unbiased alkene embedded in a 6-oxa-bicyclo[3.2.1]-oct-3-ene skeleton. We obtained exclusively a product arising from 1,2-addition of oxygen and carbon (oxyalkylation) to the alkene. While oxyalkylation products have been reported when electron-rich alkenes (e.g. enol ethers) are employed, examples derived from electronically-unbiased alkenes are rare. Here, we establish that the oxyalkylation is general for a range of 6-oxa-bicyclo[3.2.1]-oct-3-ene substrates and show that these products form competitively in the cyclization of simpler α-diazo β-ketoesters. Our data suggest increasing charge separation in the transition state for the addition promotes the oxyalkylation pathway.

Brønsted acid catalyzed [4 + 2] cycloaddition for the synthesis of bisbenzannulated spiroketals with antifungal activities

The intermolecular [4 + 2] cycloaddition of o-hydroxy benzyl alcohols with isochroman ketals was realized by CF3CO2H catalysis. A broad range of bisbenzannulated [6,6]-spiroketals were formed under the metal-free mild conditions in moderate to excellent yields (45-98%) with mostly excellent diastereoselectivities (up to >20 : 1 dr). Furthermore, the enantioselective version was also preliminarily investigated and the bisbenzannulated [6,6]-spiroketal was obtained with 61% ee in the presence of Sc(OTf)3/Feng's chiral N,N'-dioxide ligand. Some of the bisbenzannulated [6,6]-spiroketal products showed good in vitro antifungal activities against Sclerotinia sclerotiorum and Rhizoctonia solani.

Stereoretentive Catalytic [3 + 2]/[3 + 3]-Cycloaddition of Nonracemic Donor-Acceptor Cyclopropanes: Synthesis of Substituted Pyrrolidines and 1,2-Oxazinanes

A highly enantioselective preparation of substituted pyrrolidines and 1,2-oxazinanes has been achieved via stereoretentive [3 + 2]/[3 + 3]-cycloaddition of nonracemic donor-acceptor cyclopropanes with imines, triazines, and nitrones in good to high yields with broad scope under mild reaction conditions. In comparison with the well-documented approach to donor-acceptor cyclopropane reactions using racemic cyclopropane reactants and a catalyst with chiral ligands, this report features applications of enantioenriched donor-acceptor cyclopropanes as cycloadduct reactants with achiral catalysts.

(Z)-5-Benzylidene-4-phenyl-2-(p-tolyl)-4,5-dihydrooxazole

By strategic use of the valence difference between hard gold(III) and soft gold(I) catalysts, one-pot synthesis of (Z)-5-benzylidene-4-phenyl-2-(p-tolyl)-4,5-dihydrooxazole (15) from propargylic alcohol (9) and p-toluamide (13) was achieved via gold(III)-catalyzed propargylic substitution followed by gold(I)-catalyzed cyclization. The structure of 15 was confirmed by X-ray crystallographic analysis.

Ir-Catalyzed Asymmetric Cascade Allylation/Spiroketalization Reaction for Stereoselective Synthesis of Oxazoline-Spiroketals

An asymmetric cascade allylation/spiroketalization reaction between 2-(1-hydroxyallyl)phenols and 5-methyleneoxazolines is accomplished by using a chiral Ir(I) catalyst derived from commercially available iridium precursor and the Carreira ligand. This protocol furnishes a class of structurally novel and unique oxazoline-spiroketals in up to 86% yield, >99% ee and >20:1 dr. Moreover, control experiments reveal that 4,4-disubstitution on 5-methyleneoxazolines is necessary to avoid the aromatization and for the spiroketalization to occur. On the basis of this, a plausible reaction mechanism is illustrated.

Recent developments in one-pot stepwise synthesis (OPSS) of small molecules

One-pot synthesis is an active topic in organic chemistry due to its intrinsic advantages of simple operation, high mass efficiency, low cost, and less amount of waste disposal. Among three kinds of one-pot syntheses, 1) cascade reactions, 2) multicomponent reactions (MCRs), and 3) one-pot stepwise synthesis (OPSS), OPSS could be more flexible and practical since it is carried out stepwisely and have variable reaction conditions for different steps. This perspective article uses selected examples to highlight the recent development in OPSS involving cyclization, cycloaddition, rearrangement, and catalytic reactions for the synthesis of heterocyclic scaffolds, asymmetric molecules, natural products, and bioactive compounds.