Rigid cyclic systems

Optically pure P-ethanolamines react with dichlorocarbene under phase-transfer catalytic conditions to produce epoxides of high configurational retention [30]. Initial reaction occurs at the tertiary nitrogen centre (Scheme 7.29) with subsequent cleavage of the C-N bond. The reaction is configurationally controlled, as shown by the reaction of the conformationally rigid cyclic systems epoxide formation occurs with the equatorial hydroxyl system (50%), but not with the axial hydroxyl compound. 

By this procedure, the magnitude of an SCS value may indicate the relative orientation of atoms i and X in space. It has to be noted, however, that this method is valid only if the conformation of the parent molecule (R-H) is not changed significantly by the introduction of X, a condition which is often met, especially in rigid cyclic systems. 

Finally, in any rigid cyclic system like this, conformational effects are marked, and often completely control the course of reaction. 

I.5.2.4.5. Although first reported in 1970, the Sn2 addition of organocopper reagents to vinyl epoxides was not further investigated until nearly a decade later. In the past 10 years, research efforts have concentrated primarily on rigid cyclic systems, which exhibit a pronounced preference for anti-Ss2 reactivity. The stereospecificity of the reaction has been exploited to introduce steroidal side chains with the proper exocyclic stereochemistry. ... 

Oxadi-TT-methane rearrangements have been observed by both direct and triplet-sensitized excitation. The reaction is generally associated with the tt-tt excited state, but there are also examples that involve other levels. The most favorable cases for the rearrangement involve conjugation at the y-carbon (e.g., aryl) and disubstitution or a bulky substituent at the a-carbon. The reaction is also favored by more or less rigid cyclic systems. The features are present in reactants such as 8 and 9, which give particularly high yields. 

Regio- and stereoselective hydrogenolysis of allylic formates in rigid cyclic systems offers a useful method for stereoselective construchon of ring junctions. As... 

Cis Eff2 eliminations have been observed in rigid cyclic systems e.g.,... 

Particularly with rigid cyclic systems and in intramolecular versions of the ring-fission, orbital overlap conditions can play a decisive role as far as regioselectivity is concerned. 

Up to this point, we have discussed only reactions of epoxides in comparatively rigid cyclic systems, but for the preparation of homochiral starting materials the results with acyclic systems merit a closer inspection. 

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