Rule of diaxial opening

Strong preference for chair over twist boat development appears to be general, and is often referred to as the rule of diaxial opening or the Furst-Plattner rule. Twist boat processes can be encouraged by suitable substitution, but in the absence of such special features there are no known examples of cyclohexene oxides which give appreciable amounts of twist boat opened products (sometimes called diequatorial-opening products). The lone apparent exception has been shown to be an artifact caused by an impurity. 

Essentially the same transformation can be conducted on a more highly substituted steroid. The required nitroso ester can in principle be prepared from reaction of DHEA 3-acetate (6-1) with bromine and nitrous acid (Scheme 3.6). The reaction sequence would start by addition of bromine from the open a-side of the steroid to form bromonium ion 6-2. Attack on that ion by the nitroso anion from the [3 side together with the rule of diaxial opening of a three-membered ring will the lead to formation of the nitrite 6-3. Irradiation of a toluene solution of the nitrite ester with a mercury vapor lamp walks the nitro group across the gap, in effect functionalizing C19. Heating the initially formed nitroso dimer in propan-... 

A chemo- and regioselective conversion of epoxides to carbonyl compounds in 5 m LiC104/ether was reported by Sankararaman et al. [20] The stereoselectivity in the case of limonene oxide 20 can be explained by invoking the rule of diaxial ring opening (Scheme... 

Nucleophilic attack on oxirane carbon usually proceeds with inversion of configuration (Scheme 44) as expected for Sn2 reactions, even under acid conditions (Scheme 45). Scheme 45 also illustrates the fact that cyclohexene oxides open in a fran5-diaxial manner this is known as the Fiirst-Plattner rule (49HCA275) and there are very few exceptions to it. 

Treatment of an epoxide, namely methyl 2,3-anhydro-4,6-0-benzylidene-oi-D-allopyranoside (78, Scheme 21), with nitryl iodide using methanol as the solvent has been reported.123 Rather than the expected nitration, opening of the epoxide occurred to afford 30% of methyl 4,6-0-benzylidene-3-0-methyl-oi-D-glucopyranoside (79), 10% of methyl 4,6-0-benzylidene-2-0-methyl-a-D-altropyranoside (80), and a water-soluble material. The water-soluble material was later determined to be a mixture of the 2-0- (81) and 3-0-methyl (82) products, with the benzylidene acetal removed, a result consistent with a later study.124 Performing the reaction in the absence of the silver salt resulted solely in formation of the water-soluble product. The preponderance of the diequatorial product 81 is in violation of the Flirst-Plattner rule, which predicts more of the diaxial product. The authors explanation for the anti-Fiirst-Plattner addition is as follows. 

The proposed mechanism of the process includes a quasi-Favorskii rearrangement, as illustrated in Scheme 73 for the transformation of 117 into 118. Attack of bromide on the epoxide is expected to occur in a trans diaxial fashion, in accord with the Fiirst-Plattner rule, at the position of the epoxide distal to the electron-withdrawing hydroxyl group. The immediate result of this ring opening would be 121, which is in rapid conformational equilibrium with 122. Migration of a o bond from this latter conformation with concomitant expulsion of bromide would give 123, which dehydrates to afford the observed product... 

The diaxial trans-g yco structure in a-muricholic acid was substantiated by partial synthesis. a-Muricholic acid could be derived from either the 6a, 7a-epoxide XVI [Hsia et al. (39)] or the 6,9,7/9-epoxide XV [Hsia et al. (41)] (Fig. 4) by scission of the epoxide rings and hydrolysis of the acetates thus formed. In accordance to the Fiirst and Plattner rule [Fiirst and Plattner (42)], ionic opening of an ethylene oxide results in the diaxial trans-g yco. The structure of a-muricholic acid therefore should be 3a,6,9,7a-tri-hydroxy-5/9-cholanic acid (XVII). The orientation of the epoxide ring in XV and that... 

The observation that the methylative ring opening of the cis-epoxide 56 did not, according to the Furst-Plattner rule, take place at C3 (trans-diaxial process) but at C4 to provide 58 could be explained by the less stable but chelation-supported 56a. To prove this effect, the reaction was reran but this time in the presence of a crown ether to sweep all the cations from the solution. 

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