Esters hindered aryl

Entry 2 shows an E-enolate of a hindered ester reacting with an aldehyde having both an a-methyl and (3-methoxy group. The reaction shows a 13 1 preference for the Felkin approach product (3,4-syn) and is controlled by the steric effect of the a-methyl substituent. Another example of steric control with an ester enolate is found in a step in the synthesis of (-t-)-discodermolide.99 The E-enolate of a hindered aryl ester was generated using LiTMP and LiBr. Reaction through a Felkin TS resulted in syn diastereoselectivity for the hydroxy and ester groups at the new bond. 

The O-methylated extract was derivatized with 99 percent C-enriched methyl iodide. The NMR spectrum of the extract showed a strong absorption at 56 ppm (relative to TMS), which can be attributed to unhindered aryl methyl ethers (67%), a smaller absorption at 60 ppm, attributable to hindered aryl methyl ethers (23%), and a resonance at 51 ppm, assignable to methyl esters derived from carboxylic acids (10%).(16) TTiese results are consistant with those of Liotta l al, who studied the alkylation of whole Illinois No. 6 coal.(12)... 

Chaumeil, H. Signorella, S. Le Drian, C. Suzuki cross-coupling reaction of sterically hindered aryl boronates with 3-iodo-4-meth-oxybenzoic add methyl ester. Tetrahedron 2000, 56, 9655-9662. 

Ketones can also be obtained by treatment of the lithium salt of a carboxylic acid with an alkyllithium reagent (16-28). For an indirect way to convert carboxylic esters to ketones, see 16-82. A similar reaction with hindered aryl carboxylic acids has been reported. " Treatment of a p-amido acid with two equivalents of M-butyllithium, followed by reaction with an acid chloride leads to a p-keto amide.Carboxylic acids can be treated with 2-chloro-4,6-dimethoxy[l,3,5]-triazine and the RMgX/Cul to give ketones. " ... 

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

Anti-selective aldols of lithium enolates of hindered aryl esters... 

Deprotonation of ethers is another route to the a-alkoxy anions, but this pathway is often precluded by a kinetic barrier. Unless the a-carbon is benzylic [175], surmounting this barrier usually requires conditions that are not favorable to the survival of the anion [164]. Notable exceptions are the hindered aryl esters studied by Beak [176], Figure 3.13a, and the carbamates studied by Hoppe [177], shown in Figure 3.13b. In both cases, ec-butyllithium is required for deprotonation, and the carbonyls which direct the metalation by a complex-induced proximity effect [178] must be shielded from the base by large alkyl groups. Once formed, the organo-lithiums are chelated and stabilized by the heteroatom-induced dipole [179]. 

In their continuing studies Heathcock and his co-workers have reported that preformed lithium enolates of hindered aryl esters condense with aldehydes to give predominantly tf rco-/3-hydroxy-acids after hydrolysis. Optically active hydroxy-acids have been prepared from optically active propargyl alcohols, which are readily available in high optical purity by reduction of propargyl ketones with /3-3-pinanyl-9-borabicyclo[3.3.1]nonane. ° A 1,6-eliminative epoxide cleavage provides an effective synthesis of a naturally occurring aromatic hydroxy-acid, which is a metabolite of ibuprofen. ... 

Phosphite and phosphonite esters act as antioxidants by three basic mechanisms depending on their structure (1). Basically, phosphites and phosphonites are secondary antioxidants that decompose hydroperoxides. Their performance in hydroperoxide decomposition decreases from phosphonites, alkyl phosphites, aryl phosphites, down to hindered aryl phosphites. Five membered cyclic phosphites act catalytically by the formation of acidic hydrogen phosphates. In contrast, hindered aryl phosphites are interrupting the branched kinetic chain. 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

Asymmetric reductions. The reagent can effect asymmetric reduction of alkyl aryl ketones and unhindered dialkyl ketones in high optical yield.1 It is the most useful reagent known to date for asymmetric reduction of even hindered a-keto esters to (S)-a-hydroxy esters in >90% ee.2 It is also effective for asymmetric reduction of phosphinyl imines of dialkyl ketones, RlR2C=NP(0)(C6H5)2 (50-84% ee).3... 

---