Tandem reactions stereochemistry controlled

A simple, efficient multicomponent sequence was recently developed to synthesize a-hydroxyalkylpiperidine derivatives from a 4-boronohydrazonodiene, maleimides and aldehydes (Scheme 9.36) [78]. The high diastereoselectivity of these tandem reactions can be explained by a complete endo-selectivity in the first step and a cyclic chair-like transition state in the allylboration reaction. The absolute stereochemistry of the final products can be controlled by using an optically pure l-aza-4-borono-l,3 butadiene. This multi-component reaction can also be realized on a solid support, using an N-arylmaleidobenzoic acid functionalized resin. Very recently, this process... 

Sibi and Chen [42] reported a related tandem intermolecular nucleophilic free-radical addition-trapping reaction of enoate 168 establishing chirality at both a and /(-centers with control over both absolute and relative stereochemistry (Scheme 9.30) using a Lewis acid catalyst and the bisoxazoline ligand 169. They observed... 

The use of perfluoroalkyliodide in group transfer tandem additions has been examined by Wang and Lu for the preparation of butyrolactones [95T2639]. The mild reaction conditions, high chemical yield, and excellent control of alkene stereochemistry are the highlights of this methodology. 

The stereoselection in the cyclization of each diastereomer was examined independently. The stereochemical outcome of the cyclization should be predictable based on our assumption regarding the relationship between enolate stereochemistry and cyclopropane stereochemistry, the principles of asymmetric, intermolecular alkylation of optically active amides (9-13) and the assumption that the mechanism of cyclopropane formation involves a straightforward back-side, %2 reaction. In the case of the major diastereomer, the natural tendency of the enolate to produce the cis-cyclopropane will oppose the facial preference for the alkylation of the chiral enolate. Consequently, poorer stereochemical control would be ejected in the ring closure. In the minor diastereomer these two farces are working in tandem, and high degrees of stereocontrol should result. 

Anti stereochemistry in six-membered rings Conformational control from a chiral centre in the cyclohexenone Remote stereochemical control in five-membered rings prostaglandins Regio- and stereochemical control in open chain compounds Asymmetric induction by a chiral auxiliary on the enolate Tandem Michael-Michael Reactions One Conjugate Addition Follows Another Double Michael or Diels-Alder reaction ... 

The stereochemistry is controlled by the first conjugate addition represented mechanistically as 50a that gives the new lithium enolate in the right conformation for the second conjugate addition 51 giving the enolate 52 of endo-49. We shall assume that these reactions are tandem Michael-Michael additions for the rest of this section. 

Yamamoto and coworkers reported a tandem highly enantioselective 0-nitroso aldol-conjugate addition reaction catalysed by 5a. Cyclic enones and aromatic nitroso compounds afforded the corresponding bicyclic products with excellent enantioselectivity (Scheme 9.49). The results considerably extended the control of regio- and stereochemistry for the synthesis of nitroso Diels-Alder adducts. 

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