Synthesis and kinetic study of (I?)-( +)-1 ,l-dideuterio-2-(cyclohex-3-enyl)ethene (labeled 4vinylcyclohexene) has revealed new reactions of racemization and deuterium exchange, the former of which fixes the heat of formation of cis,trans-(and/or cis,cis) octa-3,6-diene-l,7-diyl diradical more firmly than heretofore. Redetermination of the kinetic parameters for the thermal reactions of cycloocta-l,5-diene establishes the heat of formation of the cleavage of the trans,trans (or, less probably, the cis,cis) diradical to butadiene to be 81.1 5 2.0 kcal/mol, in good agreement with that (82.3 kcal/mol) deduced for the same reaction in the transformations of trans-divinylcyclobutane investigated by Hammond and DeBoer. In relation to the transition state for the Diels-Alder reaction of butadiene with itself to give Cvinylcyclohexene, the cleavage of the diradical appears, within the present limitations of experimental results, to be somewhat higher in energy. The mechanism of this Diels-Alder reaction is tentatively concluded to be concerted, perhaps two stage, but not two step.ontroversy over the mechanism of the Diels-Alder C reaction has centered on the alternatives of a onestep, concerted process and a two-step, unconcerted process. The former, allowed by orbital symmetry,2 envisages the simultaneous formation of both bonds (A),3 whereas the latter involves the rate-determining formation of one bond, followed by a closure of the second bond more rapid than internal rotation (B).435To these widely disputed alternatives, Woodward and Katz have added a third, a merger of the two mechanisms into a two-stage process (C)I (Figure 1).One generally applicable approach, particularly appropriate to the study of mechanism among thermal reorganizations, involves the placement of the interrelated isomeric elements of the system on an energy surface with a view t o including or excluding certain

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The divalent tin enol ether of the racemic complex between N‐BOC‐α‐aminoheptadienone and tricarbonyliron reacts with both enantiomers of protected lactaldehydes to yield predominantly one optically active, easily isolable ketol diastereoisomer (45%). From the enantiomerically pure (S)‐(+) complex and (R)‐(+)‐tert‐butyldimethylsilyloxylactaldehyde, the major ketol is obtained almost exclusively (isolated yield 86%). From there, the multiply protected 3‐amino‐3,6‐dideoxy‐D‐aldohexose mycosamine is obtained in a few high‐yield steps (decomplexation, stereospecific reduction to an anti‐1,3‐diol, transformation into a diacetate and ozonolysis; absolute configurations S,S,S,R). Reduction of the ketol before decomplexation completely reverses the stereochemistry of the reaction [control by the Fe(CO)3 and not by the hydroxy group → syn‐diol], also giving access to the (R,S,S,R) series (configuration of the N,N‐dimethylated mycaminose). The key structures were determined by X‐ray diffraction.