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Relaxation of strain in small ring molecules is reviewed in terms of two mechanisms – σ-relaxation and π-relaxation. The σ-relaxation occurs in in-plane interaction of the ring σ bonds. Previously proposed σ-aromaticity and surface delocalization are briefly discussed. The geminal interaction theory for the σ-relaxation of ring strains is discussed and applied. The geminal interaction is less antibonding or more bonding with decrease in the bond angle, when the hybrid orbital has low s-character for the ring bonds or has an s-rich lone pair (lone pair effect). π-Relaxation of unsaturated ring molecules results from the cyclic delocalization of π electrons through the σ bonds on the saturated ring atoms. The two mechanisms of the σ- and π-relaxation are shown to be necessary for better understanding of ring strains.

An efficient and stereospecific ring formation from unsaturated alcohols, which in turn are prepared by regio- and stereospecific ring opening of acetals, is described

A novel method for the highly regioselective deprotonation of the indole alkyl substituents has been established by the particular sequence of base treatments. This success may be due to the aromatic stabilization of cross [6e/4p] conjugated systems. We are presently pursuing further application revolving around this methodology

1-Oxa-2,3-cyclohexadiene (I) can be readily generated by treatment of 5-bromo-3,4-dihydro-2H-pyran with potassium tert-butoxide in the presence of 18-crown-6 and can be trapped with dienes or olefins. Thus, the treatment of I with furan gave 50% yield of Diels-Alder adduct II, whereas with 1,1-diphenylethylene I gave 43% of the [2+2]cycloadduct III. The remarkable regio- and stereoselectivities with which [4+2] and [2+2] adducts are formed suggests a concerted, non-radical cycloaddn. mechanism. [on SciFinder (R)]

Stereospecific annulation of hydroxy vinyl ethers in the presence of triflic anhydride and tertiary amines and its synthetic application are described. Each 1-hydroxy-2-oxabicyclo[n.4.0]alkane, 2-oxabicyclo[n.4.0]alk-1(6)-ene and 2-oxabicyclo[n.4.0]alk-1(6+n)-ene is stereoselectively synthesized from the same hydroxy vinyl ether depending on the choice of the reaction conditions (temperature, solvent and tertiaryamine). These compounds lead to polyfunctionalized cyclic compounds and some natural products. We propose that this annulation reaction proceeds through a pure SN2-like mechanism.

We analyzed the bond interactions at the transition state of the Cope rearrangement of 1,5-hexadiene to disclose the significant participation of the geminal σ bonds at the reacting centers. The electron-donating σ bonds at the Z-positions and the electron-accepting σ bonds at the E-positions in the 1-substituted 1,5-hexadienes were predicted from the geminal bond participation to enhance the reactivities. For the reverse reactions, the 3-substituted 1,5-hexadienes were similarly predicted to rearrange more readily into the 1-substituted ones with the electron-donating σ bonds at the Z-positions. The predictions were confirmed by the density functional theory calculations.

The σ-bonds geminal to the π-reacting center have been proposed to participate in the organic reactions via the cyclic transition states and to control the reactivity and the selectivity. An electr...

This paper will review the current state-of-art of organoaluminum- or organoboron-mediated asymmetric reactions in the natural product synthesis. The newly developed methods heavily depend on the characteristic features of main group organometallic compounds. Asymmetric Diels-Alder reaction, pioneered by Walborsky (ref. l), has been established as one of the most important tools in modern synthesis of natural products. In the study of the influence of concave-convex topological features on asymmetric Diels-Alder reactions, a number of fascinating chiral auxiliary groups were reported (ref. 2). However, readily available dimen- thy1 fumarate appears to deserve careful reinvestigations since its primitive topological feature seems to be underestimated. Thus, the Lewis acid coordinated fumarate is considered to exist in the s-trans form predominantly and the two menthyl groups should cooperatively cover the re face of the molecule. A series of dienes was then subjected to Diels-Alder reaction with or- ganoaluminum reagent at low temperature and all the attempted reaction proceeded with excel- lent stereoselection (ref. 3).

A method is described for regioselective monoprotection of 1,n-diols. The new process depends on regioselective cleavage of orthoester, which is prepared in situ from 1,n-diols and trialkylorthoesters. The produced mono-acetal is useful in subsequent steps as a normal acetal type protecting group.

Employing the homochiral acetal template, asymmetrization of meso-substituted cyclohexanones is explored. By the use of optically-active 2,4-pentanediol as a chiral protecting group, highly diastereoselective acetal cleavage is achieved when treated with organoaluminum reagent. Dialkylaluminum amides are also effective reagents for this reaction.