Allene, 1,3-dimethyl cycloaddition reactions

The functionalized allene, DIMETHYL 2,3-PENTADIENEDIOATE, the first in the series, is an intriguing substrate for various addition and cycloaddition reactions. Finally, a new reagent, DI-ferf-BUTYL DICARBONATE, for he formation of A-f-BOC derivatives which eliminates the use o the hazardous fert-BUTYL AZIDOFORMATE (WARNING) is intrqduced. 

Lewis acids, such as EtAlCL, GaCb and AlBrs, catalyze the cycloaddition of allenes to alkenes to give methylenecyclobutanes 151. 1-Alkenes and sterically hindered olefins, such as 4,4-dimethyl-2-pentene, do not react, and from butadiene only polymeric products are obtained. Substituted olefins react more readily, i.e. 1,1,2,2-tetraalkylethylene > 1,1,2-trialkylethylene > 1,2-dialkylethylene. The acid catalyzed [2+2] cycloaddition reaction most likely proceeds via a vinyl cation. The slightly exothermic reaction proceeds at room temperature in chlorobenzene and EtAlCb was used as a catalyst in the examples listed in Table 6.7... 

The cycloaddition reaction of polar allenes with enamines also affords [2-1-2] cycloadducts. Tliese reactions also proceed via intermediate switter ionic adducts. For example, Me2C=C(H)NR2 (R = Me, -( 112)4) reacts with l,l-dimethyl-3,3-dicyanoallene to give the cycloadducts 201 and 202, resulting from addition across the polar substituted double bond of the allene. Upon long standing or heating of the initially formed cycloadducts a new cycloadduct 204 is obtained, most likely formed via the linear intermediate 203. ... 

Other approaches to (36) make use of (37, R = CH ) and reaction with a tributylstannyl allene (60) or 3-siloxypentadiene (61). A chemicoen2ymatic synthesis for both thienamycia (2) and 1 -methyl analogues starts from the chiral monoester (38), derived by enzymatic hydrolysis of the dimethyl ester, and proceeding by way of the P-lactam (39, R = H or CH ) (62,63). (3)-Methyl-3-hydroxy-2-methylpropanoate [80657-57-4] (40), C H qO, has also been used as starting material for (36) (64), whereas 1,3-dipolar cycloaddition of a chiral nitrone with a crotonate ester affords the oxa2ohdine (41) which again can be converted to a suitable P-lactam precursor (65). 

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

Saalfrank, Hoffmann and co-workers performed a number of reactions with tetra-alkoxyallenes such as 196 (Scheme 8.47) [1, 41, 105, 114—116] and demonstrated that this class of donor-substituted allenes can serve as a 1,3-dianion equivalent of malonic acid. Treatment of 196 with cyclopropyldicarboxylic acid dichloride 197 produces 2,4-dioxo-3,4-dihydro-2H-pyran 198 through release of two molecules of ethyl chloride [115]. Similarily, the reaction of this allene 196 with oxalyl chloride gives 3-chloromalonic acid anhydride derivative 199. This intermediate is a reactive dieno-phile which accepts 2,3-dimethyl-l,3-butadiene in a subsequent [4+2] cycloaddition to afford cycloadduct 200 in good yield [116]. 

The stereoselectivity of the cycloaddition with respect to the allene component was learned through study of the reaction of -(-)-1,3-dimethyl-allene 69,133,136) and acrylonitrile 8>. The reaction gave four cycloadducts in approximately equal proportions, all having a predominance of the R configuration at C(2) of the ethylidenecyclobutanes. 

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