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Nylidene Insertion Into Cyclopropanone Equivalents Christopher M

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Nylidene Insertion Into Cyclopropanone Equivalents Christopher M Enantioselective Synthesis of Alkylidenecyclobutanones via Formal Vi- nylidene Insertion into Cyclopropanone Equivalents Christopher M. Poteat and Vincent N. G. Lindsay* Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695, USA. ABSTRACT: 1-Sulfonylcyclopropanols are employed here as efficient cyclopropanone equivalents in a formal vinylidene insertion process, providing the first general synthetic route to enantioenriched alkylidenecyclobutanones. The addition of an alkenyl-Grignard reagent to the cyclopropanone leads to an alkenylcyclopropanol capable of electrophilic activation by NBS, triggering a regio- and stereospecific 1,2- migration and the formation of a brominated cyclobutanone intermediate prone to elimination. The parent β-amino ketone can also be ac- cessed by one-pot aza-Michael addition to the resulting product, and activation of the alkenylcyclopropanol intermediate with other electro- philes such as HCl or mCPBA led to the controlled formation of a variety of chiral cyclobutanones and γ-lactones via alternative pathways. The ring expansion or ring-opening of strained cyclic compounds and later allowed us to access enantioenriched 4-substituted β- constitutes a key strategy in organic synthesis for the elaboration of lactams using a formal nitrene insertion reaction with simple N- complex molecules.1 The relevance of cyclobutanone derivatives in substituted hydroxylamines as reagents (Scheme 1c).11 2 this regard cannot be understated, with countless strain-releasing Scheme 1. Reactivity of various cyclopropanone equivalents and transformations now available to synthetic chemists, thus allowing application in (3+1) ring expansion processes. rapid access to a range of structurally complex and diverse scaf- folds.3 More particularly, alkylidenecyclobutanones4,5 have been (a) Wasserman’s work: Synthesis of racemic cyclobutanones from cyclopropanone hemiketals O • Low yields HO OEt BrMg HO Br2 Br employed as privileged substrates in stereospecific ring-opening or • Limited scope 4a-b THF, reflux Et2O, 0 °C • Racemic only rearrangements and constitute divergent substrates leading to 1 isolated 29% yield (2 steps) functionalized cyclobutanones via conjugate addition of heteronu- [racemic] cleophiles4c,5o-p or organometallic reagents,4d-e,5t conjugate reduc- (b) Comparison of hemiketals 1 and 1-sulfonylcyclopropanols 2 as cyclopropanone surrogates 4f-h,5c,q-r 4i • Unstable HO OEt HO SO2R • Stable, crystalline tion or hydroformylation reactions. Moreover, these species • Poorly reactive vs • Highly reactive, modular • Achiral or racemic R1 R1 • Enantioselective access have been employed as unique precursors of oxatetrameth- 1 2 4j-l yleneethane intermediates via photoinduced electron transfer. (c) Our previous work: Formal nitrene insertion β-Lactams OH O R2 While they can be accessed in racemic form by a variety of ap- 2 R NHOH HO N Al(OTf)3 N R2 proaches including the [2+2] cycloaddition of ketenes with al- t-BuOH, 28 °C [1,2] 1 O 1 1 R 5a-f O O R HO SO2Ph R lenes, the carbonylation or oxidation of alkylidenecyclopro- [>90% ee] S 3 5g-k 5l-n Ph Me 2 steps 1 R 2 3 2 panes, the rearrangement of 1-alkynylcyclopropanols or by R MgBr R R R 5o-z 2 2 NBS; Et N O 3 classical condensation or cyclization strategies, the general enan- R HO 3 R THF, -78 °C to rt [1,2] tioselective access to alkylidenecyclobutanones still remains elu- R1 R1 (d) This work: Formal vinylidene insertion Alkylidene- sive. An interesting approach to racemic cyclobutanones reported cyclobutanones by Wasserman and co-workers involves the electrophilic activation 2 and ring expansion of 1-vinylcyclopropanol, which in turn can be Cognizant of the superior reactivity of surrogates towards the addition of organometallic reagents to afford chiral tertiary cyclo- synthesized by vinyl Grignard addition to hemiketal 1, used as a 12a cyclopropanone surrogate (Scheme 1a).6,7 While cyclobutanone propanols, we envisioned that they would constitute privileged derivatives are relatively stable to isolation and storage, cyclopro- substrates for the production of optically active cyclobutanones via panones are often highly unstable and prone to multiple decompo- an analogous stereospecific ring expansion pathway. Herein we sition pathways,1d,8,9 and it is thus often more convenient to form report the first general route to enantioenriched alkylidenecyclobu- them in situ via elimination from a surrogate such as 1 (Scheme tanones via a formal vinylidene insertion process into chiral cyclo- 1b).8d,10 Due in part to the poor leaving group ability of alkoxides, propanones starting from readily accessible 1- 2 hemiketals 1 require harsh conditions to equilibrate to the cyclo- sulfonylcyclopropanols as surrogates (Scheme 1d). The addition propanone and often lead to low yields of the desired rearranged of an alkenyl nucleophile to substrates 2 followed by appropriate electrophilic activation with N-bromosuccinimide (NBS) was products, as exemplified here in a racemic cyclobutanone synthesis (29% overall yield, see Scheme 1a).6a Recently, our group reported found to trigger a fully regio- and stereospecific 1,2-migration, lead- ing to a bromocyclobutanone prone to elimination. A number of an expedient synthesis of optically active 1-sulfonylcyclopropanols 2,11 which constitute stable yet highly reactive and modular surro- chiral alkylidenecyclobutanones could be obtained through varia- gates of cyclopropanone derivatives.12 This approach is in fact the tion of the alkenyl nucleophile and cyclopropanone substituents, first general enantioselective route to cyclopropanone equivalents, and alternative activation of the allylic alcohol with HCl or mCPBA led to the controlled formation of chiral saturated cyclobutanones Scheme 2. Scope of accessible alkylidenecyclobutanones by formal and γ-lactones instead. Other applications of this chemistry docu- vinylidene insertion into cyclopropanone equivalentsa,b mented here include the synthesis of β-aminocyclobutanones via in 3 R 2 3 2 situ aza-Michael addition, the use of a β-bromo alkylidenecyclobu- MgBr R R NBS (1 equiv); R HO SO Ph 2 R2 (3 equiv) Et N (1 equiv) O 3 tanone as a cross-coupling partner, and the stereospecific formation HO 3 R THF, -78 °C to rt R1 CH2Cl2, rt of α-quaternary cyclobutanones. Considering the relevance of chi- R1 R1 ral cyclobutanones and alkylidenecyclobutanones as strained build- 2a-2g used crude 3a-3i 2,4 ing blocks in synthesis, this work should find broad applicability O O O O O O O in the elaboration of complex and biologically relevant molecules. Bn Ph Bn Bn Ph Bn Our preliminary studies focused on the addition of vinylmagnesi- 3a, 76% 3b, 73% 3c, 76% 3d, 65%c 3e, 69% um bromide to substrate 2a to yield the corresponding 1- 96% ee (96) 96% ee (95) 98% ee (>99) (rac)d vinylcyclopropanol, and identified the use of an excess organome- Me O Me O O O tallic reagent at –78 °C as crucial to the overall reaction efficiency.13 Me We also observed that NBS was a superior electrophile to trigger Ph Ph 3f, 55% 3g, 63%, 2:1 Z:E 3h, 64%, 2:1 E:Z 3i, 63%, 2:1 E:Z the subsequent ring expansion, presumably via an hydroxycyclo- 96% ee (96) 96% ee (96) 99% ee (>99) propylbromonium species, leading to quantitative NMR yields of a 2a-2g b the ring-expanded bromocyclobutanone from the crude 1- Isolated yields from . Enantiomeric excesses were determined by HPLC analysis using a chiral stationary phase (ee of starting materi- vinylcyclopropanol intermediate. This brominated product was c al 2 in parentheses). Heated to reflux for 18 h after addition of Et3N. found to be highly sensitive to elimination by mild base,14 which dRacemic substrate 2e was used. prompted us to pursue a one-pot sequence to alkylidenecyclobuta- nones 3. To our delight, optimal conditions for the overall process Scheme 3. Direct synthesis of enantioenriched β-aminoketone 4 by from 2a were rapidly identified, involving the sequential stoichio- one-pot aza-Michael addition of the succinimide byproducta,b metric addition of NBS and Et3N at room temperature to the crude O 1-vinylcyclopropanol intermediate, affording 3a in 76% isolated i. MgBr O Yb(OTf) O HO SO Ph 3 N yield from 1-sulfonylcyclopropanol 2a with complete regio- and 2 THF, -78 °C to rt (5 mol%) ii. NBS (1 equiv); 100 °C O stereospecificity (Scheme 2). Using our previously developed two- Ph 76% Et3N, DCE, rt Ph step sequence to enantioenriched cyclopropanone equivalents 2a 3a aza-Michael 4 >20:1 dr 96% ee Ph 96% ee from methyl phenyl sulfone11 and in order to explore the scope of aIsolated yield from 2a. bEnantiomeric excess was determined by accessible alkylidenecyclobutanones, diverse chiral substrates 2a-2g HPLC analysis using a chiral stationary phase were synthesized and submitted to our formal vinylidene insertion sequence. When employing vinylmagnesium bromide as nucleo- The addition of a 1-substituted alkenyl-metal reagent such as α- phile, chiral methylenecyclobutanones 3a-3e were obtained in styrenylmagnesium bromide as initial nucleophile in this transfor- moderate to good overall yields and high enantiomeric purity, tol- mation efficiently leads to an enantioenriched α-quaternary β- erating either monosubstituted (3a-3c), gem-disubstituted (3d) or bromocyclobutanone (e.g. 5), which cannot undergo further elimi- 2,3-disubstituted (3e) substrates with similar efficiency. In the case nation to an alkene following olefin activation and 1,2-migration of 3d, additional time and heat was required for the elimination to (Scheme
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