5-Membered rings oxidative degradation

Hofmann degradation of the nonnatural protoberberine 454 afforded the 10-membered ring base 455 (65%) in addition to the styrene-type compound (13%) (Scheme 92). Dihydroxylation of the former with N-bromosuccinimide in the presence of a large excess of hydrochloric acid and subsequent oxidation of the product diol 456 with periodic acid afforded the dialdehyde 457. On irradiation in tert-butyl alcohol 457 provided ( )-cis-alpinigenine (445) along with ( )-alpinigenine (441) as a result of endo and exo intramolecular cycloaddition, respectively, of the intermediate photodienol (221,222). 

In this procedure, quinine is oxidatively degraded to meroquinene esters that are subsequently cyclized to N-acylated cis-decahydroisoquinolones in excellent overall yield, while maintaining the cis stereochemistry at the ring juncture. Furthermore, with the commercial availability of quinine, high overall yields, and ease of isolations, meroquinene and subsequent products are attractive members of a practical "chiral pool . 

Synthesis from Steroidal and Alkaloidal Intermediates Steroidal intermediates have been employed in the synthesis of certain alkaloids such as dihydroconessine (58)(ref.55), one of the 3-aminoconanine bases, from the bile acid compound 3-B-acetoxybisnorcholenic acid, a substance obtained from appropriately-protected coprostanol by oxidation and two Barbier-Wieland degradations. The method consisted in converting the carboxyl into a formylamino group, the acetoxy into a tosylate which was then substituted by a dimethylamino group, reduction of the formylamino to methylamino and formation of the five-membered ring E by... 

Reversion and ring formation have been partially overcome through placement of chain-stiffening units in the silicone-backbone. Thus linear D2-m-carborane-siloxanes (11—14) with one to three trifluoropropyl moieties per repeating unit) exhibit better thermal and oxidative stability than silicones and fluorosilicones (J ). Initial degradation occurs in air about 300 to 350 C, almost 100 above that typically experienced for siloxanes and fluorosilicones. The carborane-siloxanes exhibit T s from -50 to 0 C ( 1 ). The carborane moiety also acts to inhibit formation of six-membered rings because of its size. 

Generally speaking, the electron-poor heterocycies are more resistant to oxidative degradation than are electron-rich systems - it is nsnally possible to oxidise alkyl side-chains attached to electron-poor heterocycies whilst leaving the ring intact this is not generally trne of electron-rich, five-membered systems. 

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