Oxocanes

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 South Pacific has contributed modestly to the diversity of natural products, although the Chilean coasts remain to be adequately explored. Worth noticing are diterpenes of cyclolabdane class and C,5 oxocane acetogenins (Chart 7.7.I/FA/PO). 

Fatty acid derivs. Cjcclass oxocanes Laurencia claviformis Bdrgensen 1924, Rhodoph., from Easter Is. San-Martln 1997). 

RINGS WITH ONE OXYGEN ATOM 5.19.3.1 Oxocanes and Tetrahydrooxocins... 

Oxocane (116) is a somewhat inaccessible compound. Cyclization of 7-bromoheptanol under basic conditions gives only a 5% yield of a poorly characterized product, b.p. 

Lead tetraacetate reacts with A7-octen-l-ol to give some five-membered ether (15%) product and almost no six-membered ether, but interestingly, 10-11% each of the two 2-substituted oxocanes (122) and (123) are formed (73HCA3056). However, this reaction does not appear to be of preparative value. 

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. 

The above discussion provides a background for a discussion of the conformations of various azocanes, oxocanes, thiocanes and related compounds which lack strong ring torsional constraints. 

The X-ray structure of the substituted oxocane laurencienyne, (429), shows that the ring has the BC-3 conformation (actually, the oxygen is at the mirror image 7 position of the boat-chair) and one chlorine atom is in an axial-like position. The other substituents are in unhindered positions on the ring (80TL2299,80CSC777). 

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 ... 

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