Subject acyclic olefins

J.K. Cha et al. developed a stereocontrolled synthesis of bicyclo[5.3.0]decan-3-ones from readily available acyclic substrates. Acyclic olefin-tethered amides were first subjected to the intramolecular Kulinkovich reaction to prepare bicyclic aminocyclopropanes. This was followed by a tandem ring-expansion-cyclization sequence triggered by aerobic oxidation. The reactive intermediates in this tandem process were aminium radicals (radical cations). The p-anisidine group was chosen to lower the amine oxidation potential. This substituent was crucial for the generation of the aminium radical (if Ar = phenyl, the ring aerobic oxidation is not feasible). 

The stereochemistry of the olefin metathesis has been the subject of numerous publications which tried to rationalize the behaviour of various catalysts with acyclic olefins [1-5]. It had been noticed during these studies that a cis olefin gave preferentially a cis olefin and that a trans olefin led essentially to a trans olefin. This was explained in terms of stability of the metallacyclobutanes involved in the catalytic cycle (Scheme 1) the most favoured metallacyclobutanes are those where the substituents in positions 1 and 3 are equatorial. This simple rule allowed the explanation of most experimental results and the configuration retention. 

Previous works have shown that the diacetylation of acyclic olefins or paraffins leads to the formation of 6-diketones which cyclise to pyrylium salts. 31,32 YhQ formation of 2 is striking. We have noted that a solution of monoketone 1, acetyl chloride and aluminium chloride was stable at room temperature. Consequently, we assume that the formation of diketone 2 does not involve the acylation of monoketone but requires the acylation of the /3, T-unsaturated ketone (5-acetyl-1-methylcyclopentene) or related species. We also observed the rearrangement of the -acetylated dienolate 4 in presence of aluminium chloride or titanium tetrachloride at low temperature to diketone 2. There is an increasing evidence that unsaturated groups undergo the particularly easy 1,5-sigmatropic rearrangement. Moreover, the 1,5-acyl shift was subjected to catalysis by Lewis acids. But the possibility of an intramolecular process was conclusively ruled out by a cross-over experiment with propionyl chloride (overall yield 85-90 %). 

To avoid oligomer formation, Roberts and Rainier utilized an internal rather than a terminal olefin as a precursor to the cychzation reaction (Scheme 3.57) [62]. To this goal, internal olefin 303 was synthesized and subjected to enol ether formation but with the titanium efhylidene rather than the methylidene reagent for the acyclic enol ether forming reaction. Surprisingly, this relatively minor modification resulted in the conversion of 303 into cyclic enol ether 300 [63]. No acyclic enol ether was observed. The authors argued that the relatively moderate yield was due to the instability of the products and was not necessarily a result of an inefficient reaction. 

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