Dibal, 287 MCPBA

Fig. 5. Corey synthesis ofPGE2 and PGF2Q, where DiBAL-H = diisobutylaluminumhydride MCPBA = m — chloroperbenzoic acid py = pyridine ...

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

Scheme 71. Interconversion of spirobenzylisoquinolines and dehydrohydastine (370). Reagents a, MCPBA b, (CF3C0)20, CH2C12 c, DIBAL.

BASF AG CRBPII dba DBN DBU DIBAL-H DMAP DMF DMF-DMA DMPU HMDS HMPA HMPT H-LR LDA LDE LRAT MCPBA MOM NMO NMP PCC PhH = Badische Anilin- Soda Fabrik AG = cellular retinol-binding protein type II r dibenzylideneacetone = 1,5-diazabicyclo[4.3.0]non-5-ene = l,8-diazabicyclo[5.4.0]undec-7-ene = diisobutylaluminium hydride = 4-dimethylaminopyridine = A V-dimethylformamide = A,V-dimethylformamide, dimethylacetal = 1,3 -dimethyl-3,4,5,6-tetrahydro-2( 1H)-pyrimidone = hexamethyldisilazane = hexamethylphosphoramide = hexamethylphosphorous triamide = Hoffmann-La Roche = lithium diisopropylamide = lithium diethylamide = lecithin retinol acyltransferase = m-chloroperbenzoic acid = methoxymethyl = iV-methylmorpholine oxide = l-methyl-2-pyrrolidinone = pyridinium chlorochromate = benzene... 

The iV-aminopyrrole - benzene ring methodology has been applied to a synthesis of the 9,10-dihydrophenanthrene juncusol (218) (81TL1775). Condensation of the tetralone (213) with pyrrolidine and reaction of the enamine with ethyl 3-methoxycarbonylazo-2-butenoate gave pyrrole (214). Diels-Alder reaction of (214) with methyl propiolate produced a 3 1 mixture of (215) and its isomer in 70% yield. Pure (215) was reduced selectively with DIBAL to the alcohol, reoxidized to aldehyde, and then treated with MCPBA to generate formate (216). Saponification to the phenol followed by O-methylation and lithium aluminum hydride reduction of the hindered ester afforded (217), an intermediate which had been converted previously to juncusol (Scheme 46). 

Chemicals AIBN CAN DIBAL DMAP DMSO HMPA LAH LDA mCPBA NBS NCS or PCC PDC Py TBAF TEMPO a.a -azobislisobutyronitrile) cerium(IV) ammonium nitrate diisobutylaluminium hydride 4-(dimethylamino)pyridine dimethyl sulfoxide hexamethylphosphoramide lithium aluminum hydride lithium diisopropylamide m-chloroperoxybenzoic acid A-bromosuccinimide A-chlorosuccinimide superoxide anion radical pyridinium chlorochromate pyridinium dichromate pyridine tetrabutylammonium fluoride 2,2,6,6-tetramethyl-l-piperidinyloxy free radical... 

Scheme 13 Reagents a K2C03/Me0H h AcOH/PhsP/DEAD c MCPBA d NaBH4/(PhSe)2/ AcOH/i-PrOH e DPPA/PhsP/DEAD / DIBAL-H...

Scheme 44. Reagents (i) PivCl, EtsN, Et20, 0°C -BuLi, (R)-4-benzyl-2-oxazolidinone, THE, -78°C (ii) (l-PrO)TiCl3, l-PrjNEt, acrylonitrile, CH2CI2, 0°C (iii) NaBH4, THF-H2O (iv) TBDPSCl, imid., DMF (v) DIBAL-H, CH2CI2, -78°C (vi) H2NOH-HC1, NaOAc, EtOH (vii) NaOCl aq., CH2CI2 (viii) Zn, AcOH (ix) mCPBA, AcOH (x) K2CO3, MeOH (xi) TESCl, imid., DMF TMSCl.

Scheme 6.76 (a) Piperidine/AcOH/benzene (b) a-mercaptoacetaldehyde/EtjN (c) mCPBA (d) 370°C (e) MeOH/H (f) DIBAL/THF (g) OMSO/TPAA/GHjCl ... 

An alternative solution to the same problem uses mixed vinylalanes that contain silicon in the a-position. Since these derive from a diisobutylaluminum (DIBAL)-based hydroalumination of silylalkynes, the resulting species are diisobutyl(alkenyl)-alanes. These undergo transmetalation in the presence of an NHC-Cu complex, derived from CuCb and, importantly, a bidentate, dimeric silver NHC precursor (illustrated below). Silyl moieties can be TMS or TBS, although the latter, bulkier group in the starting alane requires room temperature for 1,4-addition to ensue. Both 5- and 6-membered enones can be used representative examples are shown below. The resulting vinylsilane in each product can be easily converted to alternative functionality, such as a ketone via oxidation with MCPBA), a halide e.g., an iodide, with NIS), and an alkene yia protodesilylation with TFA). ... 

Reagents i, CBr4-PPhj ii, MeLi-ClCOjMe iii, Hi-Pd-CaCOj-quinoline iv, DIBAL v, MCPBA vi, LiCuMej vii, MgBr-CuI viii, MsCl-Py ix, LiAlH4... 

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