Bicyclic ketals

In an analogous approach explored by Dixneuf et al., a conjugated enynyl carbonate is converted into an oxolenone or a bicyclic lactone in significant yields via double carbonylation in the presence of methanol (Scheme 22) [128]. When a neighboring carbonyl group is present in the substrate, it can also participate in palladium-catalyzed cyclization-carbonylation. Indeed, 4-yn-l-ones lead to cyclic ketals that can be easily converted into 2-cyclopentenone carboxylates in an acidic medium (Scheme 22) [129]. 

Kohnle U, Francke W, Bakke A, Polygraphuspoligraphus (L.) Response to enantiomers of beetle specific terpene alcohols and a bicyclic ketal, Z Angew Entomol... 

Ozonolysis of diketone carbonate 417 in methanol afforded an almost quantitative yield of the bicyclic diene triketone hydroxy-ester 418 (119). This remarkable transformation can also be readily explained. Ozonolysis of 417 produces the tetraketone intermediate 419 followed by methanol addition to produce the hemi-ketal 420 which undergoes a retro-Claisen reaction to 421. Then, loss of carbon dioxide from 421 yields 418. Again, 420 could also undergo a Grob type fragmentation to yield 418 directly. 

Bicyclizations of structurally analogous 5-hydroxy-1-pentynyliodonium tri-flates 34 to cyclopentene-annulated tetrahydrofurans 35 have also been reported (Scheme 56) [157]. Because the strained enol ether moiety in 35 is acid sensitive, these compounds were converted to their monobenzyl ketals. In general, bicyclizations of 34 are less efficient and less versatile than those of the tosylamido analogs. 

Hydrolysis of dioxetane 223, or of ketal 10, led to an inseparable mixture of expected product 225, as well as bicyclic isomer 226. The mixture was 1 2 (225 226), and underwent standard acylation reactions to give, for example, 179 on treatment with Ac20/pyridine/CH2Cl2. Carbonates such as 181 were available from 225 upon treatment with various chloroformates in pyridine/CH2Cl2. In other words, the alcohol 225 could be funneled away from the mixture by reaction with electrophiles, providing the desired tricyclic products. The bicyclic aldehyde 10 was isomerized to obtain tricyclic ketal 228 under dehydrating conditions in the presence of an alcohol. 

Stewart T. E., Plummer E. L., McCandless L. L., West J. R. and Silverstein R. M. (1977) Determination of enantiomer composition of several bicyclic ketal insect pheromone components. J. Chem. Ecol. 3, 27 -3. 

The bicyclic ketal 61 is converted cleanly into the pyridine 62 in the presence of an alkylnitrile and TMSOMe (Equation 195) <1997JHC325>. The active reagent, boron difluoromethanesulfonate, is generated in situ. 

Condensation of derivatives of phosphabicyclodecanones 119 with ethylene glycol in the presence of PTS/ZnCl2 gave cyclic ketals 120 of the phosphabicyclodecane series <1996RJC564> (Equation 24). Oximes and semicarba-zones were prepared from bicyclic ketones <1996RJC567>. 

Bicyclic diketone monoethylene ketal (162) prepared following the procedure of Kitahara et al. [53] was converted into octalin ketal (163) in one pot sequence which involves (a) reduction with liq. NH3, (b)... 

Reduction of bicyclic ketals. Reductive cleavage of the bicyclic ketal 1 in the presence of a Lewis acid favors ds-products, regardless of the reductant. The highest dr-selectivity is obtained with (C2H5)3SiH and TiCl4. DIBAL shows high... 

Bicyclic ketals in stereocontrolled synthesis of 6 to 8-membered saturated O-heterocycles 92SL97. 

First identified in Asian elephants during a headspace analysis of volatiles collected from secretions of the musth temporal gland of adult males,156 frontalin (52) is a bicyclic ketal, which is structurally reminiscent of the male mouse priming pheromone component 3,4-dehydro-ara-brevicomin (37). Frontalin (52), Z-7-dodecen-l-yl acetate (51), was already known because of its chemosensory role in the insect world it is an aggregation pheromone in bark beetles.157 Interestingly, the ratio of the two enantiomers of frontalin (52) changes with age and stage of musth and elicits different behavioral responses.158... 

In this reaction, considerable nitration of the tosyl groups was observed. The bicycle 89 was obtained in better yield (36%) from 88 by reaction with trifluoroacetyl nitrate in DCM <1996JOC8897, 1998LJSP5831099>. Swern oxidation of the dihydroxy diazocine 90a led to the oxygen-bridged hydroxyl diazocine 91 by transannular hemi-ketalization (Scheme 16) <1996JOC8897>. 

Bicyclic Ketals Versatile Intermediates for the Stereocontrolled Construction of Cyclic Ether Derivatives. Kotsuki, H. Synlett 1992, 97. 

---