Tetracyclic homoallylic

The total synthesis of (+)-merrilactone A was accomplished by S.J. Danishefsky and co-workers. The last step of the sequence was an acid-induced homo-Payne rearrangement. The tetracyclic homoallylic alcohol precursor was first epoxidized using mCPBA. The epoxidation was expected to occur from the same face as the C7 hydroxyl group, but due to the congested nature of the C1-C2 double bond at its 3-face, the epoxide was formed predominantly on the a-face. The epoxide substrate then was exposed to p-toluenesulfonic acid at room temperature to afford the desired oxetane ring of the natural product. 

It has been shown" that isomerization of the exocyclic allylic system of the five-membered ring D of kaurenols depends on the orientation of the C(15) hydroxyl group. The total synthesis of methyl atis-16-en-19-oate, a tetracyclic diterpenoid possessing a bicyclo[2.2.2]octane skeleton, has been accomplished" using a homoallyl-homoallyl radical rearrangement process of methyl 12-hydroxykaur-16-en-19-oate monothioimid-azolide (280) as the pivotal step. Two plausible mechanisms have been presented" ... 

It has been demonstrated that the intramolecular Diels-Alder reaction is a simple and efficient entry to different tricyclic 2-azetidinones, with a six-membered ring fused to the (3-lactam nucleus. Homoallylic mesylate 60 was used for the stereoselective preparation of fused tricyclic 2-azetidinone 61 through a tandem one-pot elimination-intramolecular Diels-Alder reaction (Scheme 21) [68, 69]. In a similar way, starting from mesylate 62, elimination and intramolecular Diels-Alder reaction have allowed the preparation of enantiopure fused tetracyclic (3-lactam 63 (Scheme 22) [70]. 1,4-Cyclohexadiene 63 is prone to undergo aromatization to afford the tetracyclic p-lactam 64 containing a benzene ring, as illustrated in Scheme 22. 

The Lewis acid-promoted carbonyl-ene reaction of enantiomerically pure 4-oxoazetidine-2-carbaldehydes gave homoallylic alcohols, which have been used for the diastereoselective preparation of fused bicyclic, tricyclic and tetracyclic p-lactams of non-conventional structure 49 and 50, using tandem one-pot radical addition/cyclization or elimination-intramolecular Diels-Alder sequences <03JOC3106>. 

Various spiro[2.4]hepta-4,6-dienes with a homoallylic group attached to Cl undergo intramolecular [4 + 2] cycloaddition at elevated temperature giving tetracyclic esters in fair yield 693,1151,2027... 

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