Tricyclic alkene

Elements of this synthesis were used as the foundation for a clever synthesis of both enantiomers from a single chiral precursor 49, with a Diels-Alder reaction comprising the key step [64]. The critical concept was the recognition that the Diels-Alder reaction of 49 could proceed through two different transition states (Scheme 7), with the product from one transition state (50a) being the tricyclic alkene 44 which had been readily converted to the desired product 48 in the synthesis of the racemate (see above). The second transition state (50b) would provide intermediate 51 that, with slightly different synthetic manipulations, could be converted to the other enantiomer of 48. The full synthesis has been described in Vol. 1, Chap. 1, and so will not be reiterated here. 

Asymmetric transfer hydrogenation with a chiral ruthenium complex is an alternative option for preparation of substituted phenethyl alcohols, which are important building blocks for the agricultural fungicide, (S)-MA20565 [47]. In the enantioselective synthesis of antidepressant sertraline (50), different chiral secondary alcohols have been proposed as pivotal intermediates (Scheme 14). Reduction of the keto ester 46 catdyzed by oxazaborolidine 45 provides chiral intermediate 47 in 90% ee [48]. Alternatively, reductive fragmentation of C2-symmetric oxa-tricyclic alkene 48 with DIBAL catalyzed by a BINAP-Ni complex generates a novel intermediate 49 in 88 % yield with 91% ee [49]. 

The key steps in the first total synthesis of (+)-momilactone A by P. Deslongchamps et al. were a highly diastereoselective transannular DIels-Alder cycloadditlon and the Prdvost reaction The 3-ketolactone moiety was installed by first treating the tricyclic alkene with A/-bromo acetamide and silver acetate to obtain the trans bromoacetate with excellent diastereoselectivlty. The cIs stereochemistry of the lactone was achieved a few steps later by the intramolecular nucleophilic displacement of the bromide with the carboxylate ion on the adjacent six-membered ring. 

Scheme 5). Similarly, isocaryophyllene (66) undergoes transannular cyclization in mineral acid to (65) and to the tricyclic alkene (67 Scheme 6) in high yield.

Feringa et al.109 succeeded in resolving thioxanthene-type alkenes. A high racemization barrier (AG = 114.1 kJmol1) was found for 2,2 -dimcthylbithi-oxanthylidene (237), compared to values reported for other bis(tricyclic) alkenes (AG = 50-92 kJ mol"V8 109-112... 

One example of a reverse vinylcyclopropane to cyclopentene rearrangement has been reported during the flash-vacuum pyrolysis of tricyclic alkene 41. ... 

GpgZrGlg or related zirconocene derivatives are effective catalysts of the carbomagnesation of 1-alkenes and some bi- and tricyclic alkenes under mild reaction conditions (<25°G). The reaction of aliphatic 1-alkenes with EtMgX is highly regioselective (Scheme 3.28), with internal double bonds being unreactive 1-alkenes with functional groups (e.g. OR, OH, SR, NR2, TMS) on the chain react similarly and allylic alcohols and ethers are active substrates. Addition to 1-alkenes with... 

The conjugated trienes cis. trims,cis-1,3,5-cyclodccalriene (16) and cisjrans,3,5-cycloun-decatriene (17) thermally isomerize with stereospecific formation of the tricyclic alkenes trans-tricyclo[5.3.0.04,6]dec-2-ene (18) and rrany-tricyelo[5.4.0.04-6]undec-2-ene (19) by reactions involving [1,5] sigmalropic hydrogen shifts followed by intramolecular [4 + 2] cycloadditions98. 

An alternative preparative method avoids the use of the Zn/Cu couple, using diethyl zinc instead. An example is taken from Abad s synthetic studies directed at spongiane.329 in this work, tricyclic alkene 416 was treated with diethylzinc and diiodomethane to give a 91% yield of 417. 

Oxymercuration of cyclodeca-l,2,5,8-tetraene and cyclodeca-1,2,5-triene using mercuric sulphate and acetic acid followed by reduction gave a mixture of the cis-decalin (138 43%) and the tricyclic alkenes (139a, 54%) and (139b). Dimethyl ketene reacts with allenes to give conjugated alkylidenecyclobutanones, e.g. adduct... 

This has been synthesized starting from the 2+2 cycloadduct between cyclopentenone and cyclobutene by introducing the diazo function and applying the Wolff rearrangement for building a tricyclic alkene. A further photochemical cycloaddition with a cyclopentenone derivative, followed by a new introduction of the diazo function, photo-Wolff rearrangement, and reduction of the acidic function for enabling insertion of the chain lead to the acid, after this succession of (five ) photochemical reactions (Scheme 6.19). 

The intramolecular aza-Prins cyclization of 1 to 2 proceeded well, though the desired tetracyclic 2 was only observed when base was included in the reaction medium. In the absence of base, tricyclic alkenes dominated. 

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