Cleavage of rings

Benzothiazoline, cleavage of ring, 42 polarizabilite of CS bond, 42 Benzothiazolium salts, action on anhydro-base, 43... 

It was postulated (169) that these amides are 8,8a-secobenzophenanthridine alkaloids produced by oxidative cleavage of ring B of the corresponding benzophenanthridines. The success of Baeyer-Villiger-type oxidations of the immonium bond of benzophenanthridine skeletons (168,171,172,175) indicates that this type of oxidation could be a real biological pathway. 

B) Cleavage of ring structure and change of the electrons via a four centre transitional state... 

Vitamin D3 (cholecalciferol) can be made in the skin from 7-dehydrocholesterol in the presence of ultraviolet light (see fig. 24.13). Vitamin D3 is formed by the cleavage of ring 3 of 7-dehydrocholesterol. Vitamin D3 made in skin or absorbed from the small intestine is transported to the liver and hydroxylated at C-25 by a microsomal mixed-... 

If an additional OH function is present in position 7 (131) fission of bond a and b dominates, followed by elimination of the substituent at position 1, (131)- -[132]->-[133]->[134]. A double bond in position 1,2 suppresses cleavage of ring B. The pattern was shown to be drastically changed by the presence of an ester function, thus enabling the structural determination of unknowns.64... 

The asymmetric total synthesis of the natural enantiomer (—)-nakadomarin A was completed by Nishida et al. in 2004 (Scheme 8.12) [82]. Diels-Alder reaction between siloxydiene 173 and chiral dienophile 172 (prepared from L-serine in 10 steps [83]) gave the highly functionalized key intermediate hydroisoquinoline 174, which was subjected to Luche reduction, cyclization, and HCl treatment to furnish the tricyclic intermediate 175. Compound 175 was converted to 177 via ozonolysis cleavage of ring B followed by recyclization of the unstable bisaldehyde to a five-membered ring by aldol condensation. The Z-olefin 178 was obtained from Wittig reaction of 177, and was further converted to furan 180 via peroxide 179. The... 

C a 7) ketones [28]., and in the cleavage of ring A of lanostan-g-one [31]. In each case the initial bond cleavage gave the expected more stable of the possible alkyl radicals. 

Oxidation of testosterone to oestradiol was again shown to involve the loss of the lj8- and 2/5-hydrogen atoms. Microbial degradation of the steroid nucleus includes cleavage of ring B followed by ring a. The enzyme for the latter process has been purified and shown to have a molecular weight of about 280 000 and to contain one iron atom. ... 

Reaction of 17/J-acetoxy-la,2oc-epoxy-lj3-methyl-5a-androstan-3-one (224) with acid gave an isomeric mixture of A-nor-2-oxo-steroids (225) carrying a formyl group at C(l). Treatment of this mixture with base provided exclusively the l/ -methyl-A-nor-steroid (226).The 3a-methyl-A-norandrostane derivative (230) has been prepared as shown in Scheme 16, i.e. by oxidative cleavage of ring A of the 3-keto-derivative (227) followed by Dieckmann condensation of the diester (228), and methylation and decarboxylation of (229). - ... 

Addition of the Reformatsky reagent to the A -20-keto-steroid (255) afforded the pentacyclic derivatives (256) and (257), which could be converted into various 16a-substituted 20-keto-pregnanes/ The 16/S-dimethylaminopregnane (259) has been obtained by cleavage of ring E of the pentacyclic steroid (258) (see... 

Due to the wide range of atoms which may occupy the sites in the heterocyclic ring, it is not possible to make too many generalizations about the physical and thermodynamic properties of these metallacycles. They are expected to be rather unstable due to ring strain and the existence of rather weak metal-carbon bonds. In fact, many of the reactions of these heterocycles involve cleavage of ring bonds. 

The interconversion of elaeocarpine and isoelaeocarpine presumably involves cleavage of ring b (c/ arrows in 3) to give an intermediate anion (5) which can recyclise to give an equilibrium mixture of the two alkaloids. ... 

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