Eight-membered oxocane

Cyclization of mixed acetals (13,300).4 This reaction is a particularly useful route to eight-membered cyclic ethers (oxocanes) and provides the first practical route to a natural oxocene, (- )-laurenyne (3), from an optically active mixed acetal 1. Thus cyclization of 1 followed by O-desilylation affords 2 as the only cyclic product. Remaining steps to 3 involved C-desilylation, for which only HF/pyridine is useful, introduction of unsaturation into the C2-side chain, and extension of the C8-side chain. Exploratory studies showed that unsaturation at the p- or y-positions to the cite of cyclization of 1 prevent or retard cyclization with a wide variety of Lewis acids. The cyclization is apparently more tolerant of substitution in the terminator position, C3-Q, of the oxocene. 

The conformational properties of such eight-membered ring molecules have been reviewed fairly recently (74MI51900). Cyclooctane is the archetypical molecule in this class, and the heterocyclic analogs, such as the azocanes, oxocanes and thiocanes, as well as carbocyclic derivatives, such as cyclooctanone, all have closely related conformational features and a brief overview of their conformations will now be given. 

Oxepanes and oxocanes.5 These seven- and eight-membered ring ethers can be obtained by cyclization of a-diazo carbonyl compounds induced with Rh2(OAc)4. Examples ... 

Eight-membered rings with one oxygen atom are named as oxocanes, oxocenes, or oxocines. The name A"-oxocene is frequently used for compounds with one double bond n = 2-4 indicates the position of the double bond. Oxocenes, however, are preferentially named as dihydo- or tetrahydrooxocines. The names of some selected oxocane and oxocine compounds are shown. 

The presence of a heteroatom, a C-C unit, or a carbonyl influences the conformation of medium-size rings, but the changes are often subtle. The calculated low-energy conformation of cyclooctane (174) is compared with those of cii-cyclooctene (175), the ether oxocane (176) and cyclooctanone (177). Although 175 is somewhat flattened, the conformations of the other three eight-membered rings are rather similar. ... 

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