Intramolecular oxymercuration approach

Figure 15. The intramolecular oxymercuration approach as a strategy for the octosyl acids.

Reinstallation of the fractured spiro ring can be accomplished either directly at the enone level or, more interestingly, after the ketone carbonyl has been capped as in 36. When 36 and stereoisomers thereof are individually subjected to intramolecular oxymercuration, the stereochemical course of the cyclization is dictated by the approach of Hg2+ to that surface of the double bond where coordination to a proximal axially oriented ether oxygen can operate. [31] Where 37 and 40 are concerned,... 

In preliminary studies we have found that this is also a powerful approach to the preparation of enantiomerically pure complex spiroacetals (Fig. 22 [23,24]). Intramolecular oxymercuration of 84 proceeds efficiently, although without any stereoselectivity. 

Scheme 9. Stereoselective intramolecular oxymercuration-demercuration approach (46). Reagents and conditions (a) Hg(OAc) (b) aq. NaOH NaBU, n-Bu4NOH.

The challenge to the synthetic chemist that was posed by these structures was the control of C6 stereochemistry. An early solution to this problem involved the intramolecular oxymercuration-demercuration approach, as illustrated in Scheme 9 (46,58). Thus, the stereoselective cyclization of 38 to the tricycUc cannabinoid skeleton by means of the intramolecular oxymercuration-demercuration reaction (58,61) gave the 6P-isomer predominantly (6P 6a = 85 15). The enantiomers were separated by chiral high-performance liquid chromatography (HPLC) using Chiralpak OD columns. The probable... 

Phenols add intramolecularly to Michael acceptors. " Under acidic conditions, a one-pot sequence starts with initial electrophilic acetylation of the activated aromatic ring and is followed by cyclization." With an appropriate leaving group in the /f-position (OMe. or other amines such as in the unsaturated carbonyl compound (e.g., 4) is formed. Other approaches to pyroncs include the self-condensation of protected //-hydroxy acrylates,intramolecular aldol reactions followed by condensation,thermal cycli-zations of unsaturated ()-chloro esters,and an iodo-cyclization-elimination sequence w th Michael acceptors.Oxymercuration of an unsaturated alcohol is an alternative cyclization approach to tetrahydropyrans. 

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