Cyclopentanols, rearrangement

Leaving the (retro-)aldol addition-initiated threefold anionic domino processes, we are now describing sequences which are initiated by a SN-type transformation. In particular, domino reactions based on SN/1,4-Brook rearrangement/SN reactions are well known. For example, the group of Schaumann obtained functionalized cyclopentanols of type 2-461 by addition of lithiated silyldithioacetals 2-458 to epoxy-homoallyl tosylates 2-459 in 41-75% yield (Scheme 2.106) [248]. 

Due to efforts of Conia s (equation 148) and Trost s groups access to more substituted vinylcyclopropanes has been gained operating with siloxy derivatives prepared either by Simmons-Smith cyclopropanation or using diphenylsulphonium cyclopropylide as a key substance (see Section VI.D.l). Oxidative rearrangement has led to a cyclopentanol... 

The allylic OH group is vital for the initial addition because neither 3-cyclopentenol nor a mixture of cyclopentene and cyclopentanol reacts with t-BuLi. The importance of the HO group is also seen from the reactions undergone by PhCHOMeCH=CH2, including 1,2- and 1,4-rearrangements of the metallated product. 

Allylation of the 2,3-Wittig products (167) leads to diallylic ethers (171) in which one of the allylic double bonds is part of a 1,5-diene system. These ethers undergo 3,3-oxy-Cope rearrangement to afford vinyl allyl ethers (170), which rearrange in situ by a 3,3-Claisen process to yield the ( )-dienals (169). Some typical results are summarized in Table 14. The cyclopentanols (175) were shown to arise from aldehydes (169) by an intramolecular thermal ene reaction. ... 

Akiyama s group developed an anodic oxidative decarboxylation of oxabi-cyclo [2.2.1] substrates that subsequently undergo skeletal rearrangement to yield 1,2,3-trisubstituted cyclopentanols [146, 147]. An example of this reaction which generates the carbocyclic framework of hydrindanes is shown in Eq. 99. 

Rearrangement of iminium ions derived from a. v-2-(alkylamino)-l-(l-aryl vinyl )cyclopentanols occurs with complete stereoselectivity to provide civ-fused hydroindols via chair transition... 

The stereochemical outcome of the rearrangement of iminium ions derived from /ra/ .v-2-amino-l-(l-arylvinyl)cyclopentanols (e.g., 11) indicates that out of the topographically different pathways only transition state 14 accounts for the preferred formation of 101134. 

Bicyclic Tf-butyrolactones. Cyclohexanols and cyclopentanols fused to a cyclopropane ring at the 2,3-position and substituted at the 4-position by an acetic acid group rearrange when treated with 7% aqeuous perchloric acid (methanol as cosolvent) at room temperature to bicyclic 7-butyrolactones. Examples ... 

Benedetti and coworkers have examined the intramolecular ring opening of epoxides by bis-activated carbanions, a process exemplifled by the rearrangement of pheny Isulfonyl epoxide 75 in a sodium ethoxide-ethanol medium to the phenylsulfony 1 cyclopentanol 76. This quantitative study on the effect of ring size in such cyclizations revealed similarities to the intramolecular radical addition onto alkenes [94JOC1518]. 

Scheme 4.29 Synthesis of functionalized cyclopentanols via domino epoxide ring opening/1,4-Brook rearrangement/Sfj reaction.

Marek and co-workers found that magnesium-to-zinc transmetalation of the alkoxides derived from addition of acetylenic Grignard reagents to acyl silanes promoted the 1,2-Brook rearrangement. The resulting propargyl zinc intermediates (e.g., 37), in equilibrium with the silyl allenol ethers of type 38, underwent diastereoselective carbocyclization in suitable systems, providing cyclopentanol 39, for example, after acid quench and desilylation. ... 

The Brook 1,4-rearrangement is useful in cyclopentanol synthesis. For example, Schaumann and co-workers demonstrated that lithiated or-silyl dithiane 112 was useful for construction of cyclopentanol 115. Addition to epoxytosylate 113 followed by 1,4-silyl migration provided lithiodithiane 114 for closure of the five-membered ring. ... 

Type 11 intramolecular ene reactions of unsaturated aldehydes and ketones have been extensively investigated. Ene reactions occur thermally or with Lewis acid catalysis to give 3-methylenecyclohexan-ois52.96-i09 or 3-methylenecycloheptanols.3-Methylenecyclopentanols cannot be formed in type II reactions. However, Lewis or Brpnsted acid induced cyclization of y,8-unsaturated aldehydes and ketones gives zwitterionic intermediates that lose a proton to give unsaturated cyclopentanols - or rearrange to cyclopentanones." " ... 

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