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A freshly prepared solution of (3-methylenepenta-1,4-dien-2-yl)magnesium chloride (63) (25 mL, 0.16 M solution in THF, 4.0 mmol, 1.6 mol equiv) was added slowly into a stirred solution of ZnBr2
(0.92 g, 4.1 mmol, 1.6 mol equiv) and THF (5.0 mL) at 0 °C. Once the
addition was complete, the reaction mixture was allowed to warm to 25 °C
and stirred for a further 20 min. To this was added 1E-bromopropene (19) (0.22 mL, 2.6 mmol, 1.0 mol equiv) followed by addition of Pd(PPh3)4
(0.15 g, 0.13 mmol, 0.050 mol equiv) and the reaction mixture was
stirred at 25 °C for 16 h with the exclusion of light. The resulting
solution was poured into water (0.15 L), stirred for 15 min and
petroleum ether (30–40 °C) (0.15 L) was added. The organic phase was
separated and aqueous phase was then extracted with petroleum ether
(30–40 °C) (2 × 0.10 L). The organic phases were combined, washed with
brine, dried over MgSO4, filtered and concentrated under reduced pressure (50 mbar, 0 °C). Purification by flash column chromatography (SiO2, petroleum ether (30–40 °C)) gave the title compound 10 (0.11 g, 0.92 mmol, 35%) as a colorless oil.
1. Diene-Transmissive Diels–Alder Cycloaddition Sequences of - and
Dendralene with the Prototypical Olefinic Dienophile
first synthesis of all five possible monomethylated dendralenes has
been achieved via two distinct synthetic strategies. The Diels–Alder
chemistry of these new dendralenes (as multidienes) with an electron
poor dienophile, N-methylmaleimide (NMM), has been studied. Thus,
simply upon mixing the dendralene and an excess of dienophile at
ambient temperature in a common solvent, sequences of cycloadditions
result in the rapid generation of complex multicyclic products. Distinct
product distributions are obtained with differently substituted
dendralenes, demonstrating that dendralene substitution influences the
pathway followed, when a matrix of mechanistic possibilities exists.
Dendralene site selectivities are traced to electronic, steric and
conformational effects, thereby allowing predictive tools for
applications of substituted dendralenes in future synthetic endeavors.
Scheme 2. Diene-Transmissive Diels–Alder Cycloaddition Sequences of Dendralene (1) with the Dienophile N-Methylmaleimide (NMM)
Scheme 3. Syntheses of the Five Mono-Methyl-Substituted-Dendralenes
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