Alkenes sequence-ring-rearrangement

Fig. la—d Typical alkene metathesis reactions ring-closing (RCM) and ring-opening (ROM) metathesis (a), diene cross metathesis (CM, b), ROM-RCM (c), and ROM-double RCM (d) sequences (ring-rearrangement reactions, RRM)... 

Substituted cycloalkenes usually react in the ring and not in the side chain. Internal alkenes with CH2 groups in both allylic positions yield a mixture of isomers, whereas terminal alkenes give primary alcohols as a result of allylic rearrangement. Later studies revealed, however, that the reactivity depends on both the structure of the alkene and reaction conditions.674 675 In alcoholic solutions, for example, racemic products are formed. Geminally disubstituted alkenes may exhibit a reactivity sequence CH > CH2 > CH3.675 676... 

A more complex picture was painted in a further study by Rapoport, which indicated that both the mechanism and reactivity sequence are dependent upon the alkene structure and reaction conditions 1,2-disubstituted alkenes (1) reacting via an oxaselenocyclobutane intermediate with a reactivity sequence CH > CH2 > CH3 geminally disubsdtuted alkenes (2) with a reactivity sequence CH > CH2 > CH3 and trisubstituted alkenes (3) with a reactivity sequence CH2 > CH3 > CH, ( )-allylic alcohols being the preferred products as established by Blichi types (2) and (3) reacting via carbenium ion intermediates (4) without four-membered ring closure or by unspecified cyclic transition states. Rapoport s evidence also showed the final step to occur by 5n1 or 5n1 processes and not by 5n2. Monosubstituted alkenes, particularly arylpropenes, commonly react with rearrangement. ... 

Ring expansions of appropriately a-substituted ketones via their vinyl carbinol derivatives has also been carried out under cationic conditions. The basic sequence, outlined in Scheme 66, begins with the addition of a vinyl organometallic to an a-substituted ketone, where X is carbon or a heteroatom. Departure of a leaving group Y then produces a cationic species, which may react further to form products. Two main pathways appear to be operative one is a cationic alkene cyclization, followed by a pinacol-like rearrangement while the other is a 3,3 or 3,3-like rearrangement, followed by an intramolecular alkylation of the intermediate enol. 

In the enantioselective total synthesis of p-lactone enzyme inhibitor (-)-ebelactone A and B, I. Paterson and coworkers constructed seven stereocenters and a trisubstituted alkene plus a very sensitive p-lactone ring. The backbone of their strategy applied an aldol reaction / Ireland-Claisen rearrangement sequence and used minimal functional group manipulation. The Ireland-Claisen rearrangement was performed in the presence of an unprotected ketone moiety and set a precedent for this protocol. The diastereoselectivity was 96 4, indicating highly ( )-selective silylketene acetal formation. 

Chalcogenide oxidation. The oxidation and selenoxide elimination sequence is applicable to the generation of functionalized alkenes which are prone to Claisen rearrangement. Medium-sized rings including tetrahydroazepin-2-ones are accessible in this manner. ... 

Interestingly, ethyl 2,2-dimethyl-5-oxo-5,6-dihydro-2H-pyridine-l-carboxylate (50), an aza-derivative of thiinone 34, behaves in a totally different way. On irradiation in benzene, it undergoes slow photodecomposition, while in the same solvent in the presence of 2,3-dimethylbut-2-ene, pyrroKdinone 51 is formed as the major (50%) photocycloadduct Although the authors proposed a rearrangement sequence of the intermediate biradical 52 formed by addition of the alkene to triplet excited 50, an alternative path (Scheme 16), including first photoisomerization of 50 to oxabicyclobutane 53 and then to the ring-contracted carbene 54, which then would be trapped by the alkene, cannot be excluded. 

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