1 -Acyl-2-methylcyclopentene

Cyclohexane can be acylated with acetyl chloride and AICI3 to yield l-acetyl-2-methylcyclopentene in 37% yield.127 Alkanes, such as 2-methylbutane, cyclohexane, methylcyclopentane, and methylcyclohexane, are easily acylated with a,p-unsaturated acyl chlorides in the presence of A1C13 and a hydride acceptor to afford mono- or bicyclic products after secondary transformations.128 Isoalkanes (isopentane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane) undergo diacylation and eventually form pyrilium salts under Friedel-Crafts acylation conditions with acetyl chloride or acetic anhydride.129... 

Interrupted Pauson-Khand reaction. Admitting oxygen to the system while heating the enyne complex yields l-acyl-2-methylcyclopentene as the product of an intramolecular reaction. 

The donor-acceptor acyl chloride-aluminium chloride complex is not only an electrophilic reagent but also a hydride ion acceptor (oxidizing reagent). The best hydride donors are the saturated hydrocarbons, specially those containing a tertiary carbon atom. The acylation of cyclohexane is an old and well-known reaction giving acetyl-methyl-cyclopentene and/or -cyclopentane. 26,27 the yields are low because the solvent used was either cyclohexane itself or small quantities of chloroform. in our hands, we found that the reaction carried out in methylene chloride solution ( IM) led to the l-acetyl-2-methylcyclopentene and l,3-diacetyl-2-methylcyclopentene Z in good yields. A cleaner reaction also occurred from methylcyclopentane. The ratio of ys could be changed by modification of the reaction conditions (see Table 1) (overall yields of the hydrocarbons functionalization were around 75-80 %). 

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 %). 

We observed that crotonyl chloride reacted very slowly with methylcyclopentane in presence of aluminium chloride. Consequently, with the aim of doing the acylation of methylcyclopentene, generated in situ, by alkenoyl chlorides, we slowly added one equivalent of acetyl chloride to a premixing solution of methylcyclopentane (1 equiv.), crotonyl chloride (1.5 equiv.) and aluminium chloride (2,5 equiv.) in methylene chloride. A very clean reaction occurred giving as main product the 3,3-dimethyl-4,5,6 trihydro-2II-pentalenone 6 (70 %) and as by-product the l-acetyl-2-methylcyclopentene 1 (10-15 %). The most... 

The ob tent ion of pentalenone may result from an acylation by crotonyl chloride of methylcyclopentene generated in situ. The first intermediate gives the divinvlke tone-aluminium chloride complex precursor of the Nazarov reaction followed by cyclization, then hydride and methyl migrations. In this process, the electrophilic and oxidizing reactivities of acyl chlorides are dissociated since acetyl chloride induces the hydride ion transfer and alkenoyl chloride performs the acylation reaction. 

The divinylketone 1 has been prepared and isolated by direct acylation of methylcyclopentene with crotonyl chloride at low temperature (-90 C). When a methylene chloride solution of 1 was treated by aluminium chloride, the pentalenone 6 has been obtained in low yield (40 %). Thus, the structure of the divinylketone 7 slows down the Nazarov cyclization (presence of two methyl on the C 3 -atoms and obtention of bicyclo 3.3.0. octane intermediate with strained structure). 

---