Zwitterion from a Cyclopropane with Geminal Donor and Acceptor Groups

Stereospecific [3+2] cycloaddition of 1,2-cyclopropanated sugars and ketones catalyzed by SnCl4: an efficient synthesis of multi-substituted perhydrofuro[2,3-b]furans and perhydrofuro[2,3-b]pyrans

An efficient stereospecific [3+2] cycloaddition of 1,2-cyclopropanated sugars and ketones catalyzed by SnCl₄ is described.

A combined experimental and theoretical study of the polar [3 + 2] cycloaddition of electrophilically activated carbonyl ylides with aldehydes and imines

Numerous 2,5-diaryl-1,3-dioxolane-4,4-dicarbonitriles and 2,4-diphenyl-1,3-oxazolidine-5,5-dicarbonitriles have been synthesized by [3 + 2] cycloaddition reactions between carbonyl ylides generated from epoxides and aldehydes or imines. In contrast to the use of aldehydes (3,4,5-trimethoxybenzaldehyde, piperonal, 1-naphthaldehyde, indole-3-carboxaldehyde, furan-2-carboxaldehyde, and thiophene-2-carboxaldehyde), the reactions performed with imines (N-(phenylmethylene)methanamine, N-(1,3-benzodioxol-5-ylmethylene)propylamine, N-(1,3-benzodioxol-5-ylmethylene)butylamine, and N-(1,3-benzodioxol-5-ylmethylene)benzylamine) proceed diastereoselectively. The effect of microwave irradiation on the outcome of the reaction was studied. The mechanism of these [3 + 2] cycloaddition reactions has been theoretically investigated using DFT methods. These cycloadditions, which have one-step mechanisms, consist of the nucleophilic attack of the aldehyde oxygen or imine nitrogen on the carbonyl ylide. For the reaction with aldehydes, a back-donation effect is responsible for the unexpected reverse charge transfer found at the transition structure. The analysis of the reactivity indexes indicates that the large electrophilic character of the carbonyl ylides induces them to act as strong electrophiles in these polar [3 + 2] cycloaddition reactions.

2,5-Dihydro-1,3,4-oxadiazoles and Bis(heteroatom-substituted)carbenes

2,5-Dihydro-1,3,4-oxadiazoles with heteroatom substituents at C-2 (also known as corresponding Delta(3)-1,3,4-oxadiazolines) are very useful materials for the thermal generation of acetoxy(alkoxy)-, dialkoxy-, alkoxy(aryloxy)-, diaryloxy-, alkoxy(alkylthio)-, bis(alkylthio)- and alkoxy(amino)-carbenes. Such carbenes are relatively nucleophilic and react with a variety of electrophilic functional groups. This Account reviews our work to prepare 2,5-dihydro-1,3,4-oxadiazoles and apply them to the synthesis of other target molecules. Carbenes bearing alkoxy substituents at the carbene carbon can be looked upon as acetals of carbon monoxide. Their reactions with electron-deficient alkenes or alkynes can afford acetals of cyclopropanones or cyclopropenones. Although such products may be formed by concerted cycloaddition, initial formation of one bond to produce zwitterions is likely in some cases. In addition, the reactions of these molecules with carbonyl compounds could involve acetals of alpha-lactones as intermediates. Although we have isolated some acetals of cyclopropanones, we have not isolated the other intermediates. However, some of the products we observe probably resulted from these intermediates. For example, dimethoxycarbene reacts with maleic anhydrides to generate six-membered rings by apparent carbonyl addition followed by an acyloxy group transfer. Similarly, dimethoxycarbene reacts with thiocarbonyl compounds, such as xanthates, by apparent carbonyl addition and transfer of the alkylthiyl group to the erstwhile carbene carbon. Reaction of ROC:(OR)' with an alcohol R''OH or a phenol should generate chiral (racemic) orthoformates. Similarly, reaction of alkoxy(alkylthiyl) carbenes with alcohols could generate chiral (racemic) dialkyl thiyl orthoformates. 2,5-Dihydro-1,3,4-oxadiazoles are particularly useful for synthetic applications because they are easily prepared and reasonably stable, thermal sources of bis(heteroatom)carbenes. Such carbenes react with a variety of electrophilic functional groups, often with rearrangement of initial products. Those products, many of them new, could be used as starting materials toward other synthetic targets. Isolation of products from the reaction is relatively simple because the coproducts of thermolysis of the oxadiazoles are primarily N(2) and a ketone, such as acetone. Thus, 2,5-dihydro-1,3,4-oxadiazoles should be part of every synthesis chemist's toolbox.