Ring with preceding

The potential of the method is best demonstrated when the ring opening is preceded by chemo- and stereoselective alkylation reactions. As Fig. 12 shows, two main types of alkylation events can be carried out on the designed (3-lactam precursor. Type 1 alkylation takes place at the C3 position of the (3-lactam ring, with entrance of the electrophile R group from the side opposite to the Ar group at C4. The second one (type 2) takes place at the exocyclic carbon directly bonded to the... 

Rates of the losses of methyl and methane from the molecular ion of methylcyclopentane have been determined over the time range 40 ps to microseconds [288]. The loss of ethylene from methylcyclopentane and decompositions of methylcyclohexane were also investigated. With support from I3C labelling, it was suggested that at times shorter than 1 ns, methyl was lost from the intact cyclopentane ion, but that at longer times ring opening preceded the decomposition. 

These two steps—protonation followed by nucleophilic attack—are the exact rever.se of the opening of epoxide rings with strong nucleophiles, where nucleophilic attack precedes protonation. 

In agreement with similar experiments on nicotine and hyoscyamine Datura metel) iV-methylputrescine (24) was a much better precursor for cuscohygrine and hyoscyamine in S. lurida than either putrescine or ornithine. [2- " C,(5- N]Omithine was incorporated into cuscohygrine with no loss of relative to and thus the nitrogen atom in cuscohygrine arises specifically from the 5-amino-group of ornithine. It was concluded from these data that decarboxylation and iV-methylation precede cyclisation to the N-methyl-pyrrolidine ring, with 5-Af-methylaminobutyraldehyde (25) or its cyclic form (26) as an intermediate a similar role for the latter compound has been demonstrated in nicotine biosynthesis. ... 

With these mechanisms the cis isomer 2 cannot be formed. The formation of this isomer when aluminium chloride is used as catalyst can be explained [11] by simultaneous elimination of the isopropyl group and cleavage of ring B (followed by the reformation of ring B). An alternative mechanism in which the opening of ring B precedes deisopropylation has been proposed [13]. 

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