Cyclopropylcarbinols rearrangement

Oxidative rearrangement of the tertiary-cyclopropylcarbinol 262 using PCC leads to the / ,y-unsaturated ketone 263 and halide 264 [149], (Scheme 104)... 

These electrophilic conjunctive reagents require donor reactants. The cyclopropyl-carbinols as precursors to cyclobutanones arise by simple addition of organometallics. For example, the cyclobutanone 47 derives by addition of vinyllithium to 44 followed by rearrangement with aqueous fluoroboric acid 92). In some cases, this route to cyclopropylcarbinols is preferred. Addition of 41 to aldehydes or ketones... 

Enolates also serve as suitable reaction partners in a directed aldol condensation (Eq. 52) 88) Dehydration of the aldol 51 to give enone (POCl3, HMPA then C5H5N) followed by reduction [LiAlH(nC4H9) (iC4H9)2] produces a vinylogue of a cyclopropylcarbinol 53. Aqueous fluoroboric acid smoothly rearranges 53 to the... 

A bis-vinylogue of a cyclopropylcarbinol 57 arises by standard transformations (Eq. 59) from the Peterson olefination product 56. Here too, acid induced rearrangement proceeds exclusively to the cyclobutanone and not to larger ring products93). 

A rearrangement (Scheme 43), similar to that of isopropenyl-substituted cyclopropylcarbinols (see Section 13.02.8.1.1), gives the corresponding 4,7-dihydrooxepine-2,3-dicarboxylic acid derivatives from derivatives of 5-isopropenyl-4,5-dihydrofuran-2,3-dicarboxylic acid <1997BCJ2777>. 

The addition of a-lithiovinyltrimethylsilane 151 83), generated from a-bromovinyl-trimethylsilane84,) with r-BuLi (1.5 equivalents) at —78 °C in ether, to ketones and aldehydes was also investigated. The allylic alcohols 152 thus obtained underwent smooth cyclopropanation when the modified Simmons-Smith procedure utilizing EtZnI85) was applied. The cyclopropylcarbinols 153 were directly dehydrated without rearrangement upon exposure to catalytic amounts of p-TsOH in benzene at 20 °C to give 149 in yields of 52-75%, Eq. (48) 79,8L). 

The adducts of 1-arylthiocyclopropyllithium 116 61) to aldehydes and ketones, upon treatment with p-toluenesulfonic acid in refluxing benzene under anhydrous conditions or with the Burgess reagent67), underwent ring expansion to 1 -phenylthiocyclobutenes, which may be hydrolyzed to cyclobutanones, desulfurized to cyclobutanes or thermo-lyzed to dienes. Thus, the cyclopropylcarbinol 233, adduct of 116a to /-butyl methyl ketone, was rearranged to the cyclobutanone enol thioether 234, Eq. (72) 139). 

Direct rearrangement to cyclobutanones can be achieved under four sets of conditions a) with 48% aqueous fluoroboric acid in ether, at room temperature b) with anhydrous stannic chloride in methylene chloride, normally at room temperature c) with p-toluenesulfonic acid in benzene saturated with water, at reflux and d) with trimethyloxonium tetrafluoroborate in methylene chloride (Meerwein s reagent). For instance, the cyclopropylcarbinol 233 was rearranged to 2-/-butyl-2-methylcyclo-butanone 235, upon treatment with one equivalent of pTsOH in water-saturated benzene at reflux for 1.5 hr, Eq. (72) 139). In a slightly different approach, the alcohol... 

OL-Substituted cyclobulanones. 1-Ethoxycyclopropyllithium (2) reacts with a wide variety of aldehydes or ketones to form cyclopropylcarbinols (3), usually in >90% yield. These adducts rearrange in the presence of tetrafluoroboric acid (0.5-4 equiv.) at room temperature to cyclobutanones (4). 

The oxidative rearrangement of tertiary cyclopropylcarbinols to 3,4-unsaturated carbonyl conqrounds is analogous (or homologous) to the reaction of allylic alcohols, and is shown in the example in equation... 

The oxidative rearrangement of tertiary cyclopropylcarbinols to 3,4-unsaturated carbonyl compounds is analogous (or homologous) to the reaction of allylic alcohols, and is shown in the example in equation (29). This reaction has been shown to proceed stereospecifically in the conversion of the cis-substituted cyclopropylcarbinol (29) to the (Z)-enynone (30) shown in equation (30). The substrates with R = H, Me and TMS all gave comparable yields. 

Oxidative rearrangement of cyclopropylearbinoh to, y-enoues Tertiary cyclopropylcarbinols (1), which can be prepared by addition of cyclopropyl organometallic reagents to ketones, are oxidized by PCC (5 equiv.) mainly to /3,y-enones (2). The related reagent C5H5NHCr03BF4 is equally effective. Traces... 

Ring expansion of cyclopropylcarbinol to cyclobutanol is the basis of an efficient cyclobutene synthesis (equation 25). An elegant approach to polycyclic hydrocarbons is realized by a cascade of cyclopropylcarbiny 1-cyclobutyl rearrangements (equation 26) . 

New ring-opening methods for phenylthio-substituted cyclopropylcarbinols generate intermediates with various possibilities for further transformation an example is shown in equation lOS " An overall [3 -I- 3]-annulation process is offered by combination of the Trost route to 2-vinylcyclobutanones and an alkoxy-accelerated rearrangement (equation 109). ... 

Besides ylids, carbenoids (366) can also be used since they too can condense with ketones to yield oxaspiropentanes p67) or heteroatom-substituted cyclopropylcarbinols (368), both of which rearrange to cyclobutanone derivatives (369) under acid catalysis. The 1-ethoxycyclopropyllithium is the carbenoid reagent of choice because of its ease of... 

P-hydroxyalkyl selenides bearing two alkyl substituents at the selenium-bearing carbon, a tendency to produce rearranged compounds besides the expected alkenes (Scheme 165, a). These become the exclusive compounds obtained - when l-seleno-l-(r-hydroxyalkyl)cyclopropanes are involved (Schemes 132, a Scheme 163, f-h). This behavior is general for almost all cyclopropylcarbinols bea g a heterosubstituent (e.g. OR, NR2, SR, SeR) at that position on the cyclopropane ring (see Section 2.6.4.5.3).>2... 

Vinyl cyclopropanols have been prepared by the addition of alkenyl Grignard reagents to a variety of cyclopropanone equivalents. Upon treatment with acid, the vinyl cyclopropanols rearrange to a-substituted cyclobutanones. Alternatively, a variety of a-heteroatora-substituted cyclopropyllithiura reagents have been developed. These react with aldehydes and ketones to afford cyclopropylcarbinols which also rearrange to cyclobutanones under acid catalysis.Lastly, vinyl cyclopropanols and cyclopropylcarbinols have been prepared by the cyclopropanation of enol silyl... 

As shown in the Table, a wide variety of a-substituted cyclobutanones have been prepared by the general method described here. The time required for rearrangement of the intermediate cyclopropylcarbinols varies from less than 5 min for entry 2 to 48 hr for entry 10. With most enones and enals, only 1,2-addition is observed, but in two cases (entries 3 and 4), a significant amount of the 1,4-adduct was also produced. The Increased 1,4-addition seen in entry 3 apparently occurs because of steric factors, whereas that seen in entry 4 presumably occurs because of chelation of the organo-lithium to the benzyl ether oxygen. 

R = Me). In an alternative synthesis of dendrolasin, Parker and Johnson used the highly stereoselective rearrangement of the cyclopropylcarbinol (23) as the means of attaining the irons double bond in the homoallylic bromide (24). Conversion of the bromo-group to an aldehydo-group, followed by a Wittig reaction with isopropylidenetriphenylphosphorane completed the stereoselective synthesis of dendrolasin (22, R = Me). 

---