Stereochemistry allene cycloadditions

Fluorinated allenes are especially reactive in cycloadditions because of their highly strained double bonds [118, 119] 1,1-Difluoro- and 1-fluoroallene readily undergo both [2+2] and [4+2] cycloadditions [118 124] (equations 50-52) Exten sive studies of stereochemistry and regioselectivity show that cyclobutane forma-... 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Chiral alkoxy allenes derived from 1,3-alkylidene-L-erythritol and -D-threitol have been used in cycloaddition reactions to provide the 4-substituted /3-lactams 418 (R = Me, Ph). Intramolecular alkylation at nitrogen was achieved by the action of potassium carbonate and tetrabutylammonium bromide in dry acetonitrile. The absolute stereochemistry of the product 419 (R = Me, Ph) was assigned on the basis of the CD helicity rule (see Section 2.04.3.5) and NMR spectroscopy. The [2+2] cycloaddition of CSI to threitol vinyl ethers was found to have low stereoselectivity in contrast to the findings with erythritol derivatives <2004CH414, 2005EJ0429>. 

With the hypothesis that allene-olefin cycloadditions and methylenecyclobutane rearrangements have a common intermediate comes the prediction that common stereochemical modes for the various rotations involved will prevail. The cycloaddition model outlined above involves disrotatory ring closure with the same sense of rotation about C(4)C(5) and C(5)C(2). For the methylenecyclobutane rearrangement, the same stereochemistry is anticipated for the reverse reaction. 

Additions to Cyclopentenones and Related Systems. (2 + 2)-Cycloadditions are reported following the irradiation of mixtures of alkyl and aryl 2-thioxo-3/f-benzoxazole-3-carboxylates with alkenes. Cycloaddition also occurs to the CS double bond. The photochemical additions of arylalkenes to 3-phenylcyclo-pentenone and 3-phenyl cyclohexenone have been studied. The regio- and stereochemistry observed in the additions has been rationalized in terms of the stability of the intermediate biradicals. Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C affords... 

A stereochemical model was proposed to account for the chiral discrimination, which was consistent with the absolute configurations observed in the enantioselec-tive cycloaddition reactions. As the absolute sense of stereochemistry is predictable, this reaction could be used to assign the absolute configurations of new, optically active allenes. 

Allenes and cumulenes in general are more reactive than alkenes in undergoing cycloaddition reactions with isolated non-activated double bonds. The stereochemistry of the major cycloadducts can be predicted from the most stable diradicals formed upon initial bonding of the sp carbon to the allene. The [2-1-2] cycloaddition reaction of allenes to olefins proceeds thermally, photochemically and at room temperature, using a Lewis acid catalyst. The mechanism of these reactions are considered to be stepwise processes, with radical or ionic intermediates. 

Allenes have been added to many cyclic a,/8-unsaturated ketones to give methylenecyclobutane derivatives. The stereochemistry of such additions has been rationalized by Wiesner, who has used the method extensively in the synthesis of diterpenoid alkaloids. For example, the synthesis (Scheme 10) of talatisamine (39) involved the cycloaddition of allene to the enone (35) to give the photoadduct (36) which was converted into the masked aldol (37). Retro-aldol reaction followed by another aldol cyclization gave (38) which was converted into talatisamine (39). Intramolecular photocyclizations of appropriate allenic enones can also be expected to be synthetically useful. The formation of (41) from (40) in high yield is a promising reaction. A closely related process is the cyclization of (42) to (43) (Scheme 11). ... 

Protonation of the allenyl complex (16) with HPFg at — 20°C gives the cationic acetylene complex (17), and this observation, together with product stereochemistry, strongly suggests that the [2 + 3] cycloaddition reactions of (16) with tetracyanoethylene and toluene-p-sulphonyl isocyanate, which generate (18) and (19), respectively, proceed via an acetylene dipolar ion (20) rather than an allene (21). However, protonation of the related butynyl complex (22) generates stereospecifically a cationic allene complex (23), which on... 

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