By Pauline Chiu, Mark Lautens (auth.), Peter Metz (eds.)
This quantity offers intriguing new advancements in preparative heterocyclic chemistry with a unique emphasis at the stereoselective synthesis of - or by means of - heterocycles. quite a lot of sleek equipment and techniques for the development and artificial elaboration of flexible heterocycles is surveyed intensive by means of top researchers during this box. as a result of prime quality and updated assurance of every bankruptcy, this quantity presents a worthwhile review of the various points mentioned and may whilst be hugely inspiring for the specialist man made natural chemist in addition to the non-specialist reader.
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Example text
O 0 ~ @ KH, PhMe, reflux 40% yield - 0 ~ ~ 0 . 1 Hydrogen Addition In the special case of the oxabicyclic compounds with bridgehead carbons bearing aryl substituents, hydrogenolysis results in the cleavage of the bridging 57 Using Ring-Opening Reactions of Oxabicyclic Compounds as a Strategy in Organic Synthesis carbon-oxygen bonds. In Rodrigo's synthesis of the lignans of Podophyllum,all eight diastereomers could be obtained from the common intermediate 227 [193]. 1] nucleus of 227, Eq. 139. Pelter's synthesis of (-)-isopodophyllotoxin utilized a similar hydrogenolysis strategy with an asymmetric Diels-Alder oxabicyclic adduct derived from menthol (Menth) as substrate, Eq.
Regioisomeric addition products 100 are formed from 98. 31 Using Ring-Opening Reactions of Oxabicyclic Compounds as a Strategy in Organic Synthesis E 0 O O 0 2 L 96 '; O1 X" + 0 96a 96b X J 97 1. NaOMe 2. 1]heptenes (Scheme 7). 1]octenone 85 initially forms the kinetic regioisomer 101, but upon prolonged reaction time, the formation of thermodynamically favored regioisomer 102 is also observed, Eq. 77. 5h, 13:1 t*= 4h, 8:1 t*= 8 days, 1:1 102 (77) P. Chiu. M. 2 Enantioselective Desymmetrization Reactions Asymmetric derivatization of m e s o oxabicyclic compounds generates enantiomerically enriched oxabicyclic compounds and provides a source of chiral oxabicyclic starting materials.
87% OBn OBn ] +B ~ , ~ . ~ o ~ o ~ I B .. 1] compounds have been reported but less systematically studied. Because the majority of these compounds are derived from oxyallyl cation cycloadditions, most experiments on the three-carbon bridge involve addition to the bicyclic ketone. 1] ketone 90 undergoes reduction with bulky hydride sources such as L-selectride to generate the endo alcohol, Eq. 68 [112]. Presumably, the selectivity is due to equatorial attack of the hydride at the ketone of the pyranone in a pseudo chair conformation.