PMP group

Methoxyphenyl (PMP) ethers find occasional use as hydroxy protecting groups. Unlike benzylic groups, they cannot be made directly from the alcohol. Instead, the phenoxy group must be introduced by a nucleophilic substitution.185 Mitsunobu conditions are frequently used.186 The PMP group can be cleaved by oxidation with CAN. 

The amine-catalyzed Mannich reaction has also been a subject of special reviews [243, 244]. In general, yields and enantioselectivities of proline-catalyzed Mannich reactions are very high. Initially, the reactions were restricted to imines bearing an aromatic A-substituent, such as the p-methoxyphenyl (PMP) group. This restriction considerably limited the usefulness of the protocol, because relatively... 

The six-membered transition-state is stabilized by hydrogen-bonding between the nitrogen of the imine and the carboxyl group of proline. Switching of the facial selectivity is disfavored, because of to steric repulsion between the PMP group of the imine and the pyrrolidine moiety of the enamine. This is opposite to similar direct asymmetric aldol reaction in which re-facial attack occurs [27, 30, 36]. 

The PMP acetal is quite susceptible to acid-catalyzed cleavage. In the following case a normally readily cleaved cyclopentylidene group could not be cleaved in preference to the PMP acetal. In a very creative move the authors prepared a charge transfer complex with the extremely electron-dehcient trinitrotoluene and the electron-rich PMP groups to suppress protonation... 

Benzyloxy derivative 764a can be transformed to the carbapenem antibiotic ( + )-PS-5 (758) as shown in Scheme 101. The first required manipulation is removal of the benzyloxy group in the 3-position, which is accomplished by reductive debenzylation, conversion of the resultant hydroxy group to a xanthate, and Barton deoxygenation. Next, enolate formation with LDA followed by alkylation with four equivalents of ethyl iodode gives the trans-3,4-disubstituted j5-lactam 766. Removal of the PMP group with CAN furnishes intermediate 753, which is required for completion of the synthesis of ( + )-PS-5. 

Reductive cleavage of the /tara-methoxyphenyl (PMP-) group released the primary alcohol and the compound was converted into the corresponding methyl glycoside upon reaction with methanol in the presence of PhsP.HBr [65]. Subsequently, the primary alcohol was replaced by iodine to yield 68 to pave the way for the introduction of the exomethylene functionality required for the key Perrier s carbocy-clization reaction. Carbocycle 70 was obtained after exposure of 5-enopyranoside 69 to Hg(OCOCp3)2 in acetone/acetate buffer and the subsequent p-elimination. The synthesis of the C-ring of the alkaloid was completed by 1,4-reduction and formation of the vinyl triflate 71 with the Comins reagent. 

Akiyama et al. reported a Brpnsted acid-catalyzed synthesis of 3-aryl-1-trifluoromethyltetrahydroisoquinolines 230 and 230 by a benzylic [l,5]-hydride shift-mediated C-H bond functionalization (Scheme 87) [142], which features the diastereo-divergent synthesis of 3-aryl-l-trifluoromethyltetrahydroisoquinolines 230 and 230 by tuning the substiments on nitrogen atom. The trifluoromethylketimine derived from para-anisidine and activated by Tf2NH served as hydride acceptor and the substituents on ketimines had dramatic impacts on the diastereoselectivities cis-product 230 could be furnished as major product when R was PMP group, whereas the diastereoselectivity was reversed with R as hydrogen. 

Roush and coworkers reported an intramolecular MBH on a highly functionalized intermediate which resulted in the total synthesis of (-)-spinosyn A (111). Under dilute conditions in /erf-amyl alcohol, compound 112 underwent a vinylogous MBH reaction in the presence of trimethyl phosphine to afford the tetracyclic structure 113 as the major product. Subsequent reductive dehalogentaion and cleavage of the PMP group delivered the spinosyn A pseudoglycon 114 in 73% yield from compound 112. Glycosidation of 114 and subsequent transformations yielded (-)-spinosyn A (111). 

Reductive amination combines an in situ imine formation with a consecutive reduction. If this reduction is carried out in an asymmetric fashion, chiral amines are accessible in one step from the corresponding ketones (Scheme 32.20, route A). Primary amines are available via the p-methoxy-phenyl (PMP)-group that can be removed with cerium ammonium nitrate (CAN) afterward (route B). 

In summary, lithium alkyl aryl sulfoxides can be efficiently used as equivalents of chiral a-hydroxy alkyl carbanion in the synthesis of 3-fiuoroalkyl 3-amino alcohols. On the other hand, fluorinated imines are versatile and readily available electrophilic sources of fluoroalkyl/fluoroaiyl amino groups. Excellent stereocontrol and reactivity can be achieved using aldimines A-protected with a PM group, which also makes fluorinated aldimines electronrich enough to be easy to handle. Furthermore, the PMP group can be cleaved readily and under mild conditions, thus representing the... 

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