O-acetates

Ethylenediamine, CH3CN, THE, EtOH, 60°. The phthalimide group and O-acetate are not cleaved with this reagent. ... 

N,O-acetal intermediate 172, y,<5-unsaturated amide 171. It is important to note that there is a correspondence between the stereochemistry at C-41 of the allylic alcohol substrate 173 and at C-37 of the amide product 171. Provided that the configuration of the hydroxyl-bearing carbon in 173 can be established as shown, then the subsequent suprafacial [3,3] sigmatropic rearrangement would ensure the stereospecific introduction of the C-37 side chain during the course of the Eschenmoser-Claisen rearrangement, stereochemistry is transferred from C-41 to C-37. Ketone 174, a potential intermediate for a synthesis of 173, could conceivably be fashioned in short order from epoxide 175. 

Oxidation of the methyl substituent in compounds 4 to the corresponding aldehydes and subsequent reaction with ephedrine leads to (V.O-acetals, which can be separated by crystallization into the two diastereomers. Treatment with silica gel then gives the enantiomerically pure aldehydes.17... 

The weaker Lewis add TMSOTf 20 as catalyst gives, after 2 h at 0°C in CH2CI2, a 20 80 mixture of 805 and 806 in only 23% yield (Scheme 6.8). But this yield will probably increase either on longer reaction time at 0°C or on shorter reaction time at 25 °C On replacing one of the methyl groups in 804 by an acetylene substituent the resulting enyne adds allyltrimethylsilane 82 or anisole in the presence of TMSOTf 20 to give allenes [18]. Substituted allyltrimethylsilanes such as 808 react with the allylic silylether 807 after 70 h at 25 °C in 62% yield to a 41 59 mixture of 809 and 810 as well as 7 [17]. Closely related additions of 82 to allylic ethers or O-acetates are discussed in Refs. 17a-c. 

Physiol Properties of Amino Carba-hexopyranose Penta-A/, O- Acetates... 

Treatment of N, O-acetals 42 in which the para position is unsubstituted, with a twofold excess of the corresponding amine HC1 in boiling 50% aqueous methanol gives rise to the bis(4-aminoaryl)methane derivative.62 43. This method is suitable only for 4-AT,AT-unsubstituted diarylamines (Eq. 5). 

Another example of an intramolecular cyclization initiated by reactions of an acyliminium ion [32] with an unactivated alkene has been published by Veenstra and coworkers. In their total synthesis of CGP 49823 (1-116), a potent NK antagonist [33], these authors treated the N,O-acetal 1-112 with 2 equiv. of chlorosulfonic acid in acetonitrile to afford acyliminium ion 1-113 (Scheme 1.29) [34]. This is qualified for a cyclization, creating piperidine cation 1-114, which is then trapped by... 

It is assumed that the overall process is initiated by a Michael addition of the 1,3-dicarbonyl compound onto the ci,(5-uri saturated carbonyl derivative. There follows the formation of either an aminal and an iminium intermediate which is followed by the formation of two N.O-acetals. 

The formation of an iminium ion as 2-530 is also proposed by Heaney and coworkers in the synthesis of a tetrahydro- 3-carboline 2-531 (Scheme 2.120) [282]. Herein, heating a solution of tryptamine (2-526) and the acetal 2-527 in the presence of 10 mol% of Sc(OTf)3 gives in the first step the N, O-acetal 2-528, which then leads to the lactam 2-529 and further to the iminium ion 2-530 by elimination of methanol. The last step is a well-known Pictet-Spengler type cyclization to give the final product 2-531 in 91% yield. 

Treatment of 2-aroyl-3-aminoacrylates 442, or their 0-acetate derivatives <1996USP5539110> with KF, or with another base, yielded 7-oxo-2,3-dihydro-7f/-pyrido[l,2,3- i ]-l,4-benzoxazine-6-carboxylates or 6-carboxylic acids (e.g., <1996GEP4428020, 1997H(45)137>). Cyclization of 2-aroyl-3-aminoacrylates 443 <1997H(45)137> or their O-acetates <1996USP5539110> led to pyrido[l,2,3- i ]-l,4-benzoxazine-6-carboxylic acids 444 (Equation 84). 

In reactions of chiral aldehydes, TiIV compounds work well as both activators and chelation control agents, a- or A-oxygcnated chiral aldehydes react with allylsilanes to afford chiral homoallylic alcohols with high selectivity (Scheme 22).85 These chiral alcohols are useful synthetic units for the synthesis of highly functionalized chiral compounds. Cyclic chiral 0,0- and A/O-acetals react with allylsilanes in the same way.86,87 Allenylsilanes have also been reported as allylation agents. 

More recently, the prostanoid metabolites of P. homomalla have been reinvestigated relative to their potential natural function [69], A minor new metabolite was isolated and characterized in this work by high field NMR studies. Its structure, the 9-O-acetate of methyl PGF2ar (55), seems secure given the opportunity to compare H and 13C NMR data sets with an earlier isolate of the 11-O-acetate of PGF2c( (56) [70],... 

Frankincense, also known as olibanum, is obtained from trees belonging to the genus Boswellia (Burseraceae family). It is one of the best-known ancient plant resins. The ancient Egyptians were the first to use it as incense in embalming practices and in the preparation of medicines, cosmetics and perfumes, and today it is still used therapeutically. It contains pentacyclic triterpenoids belonging to oleanane, ursane or lupane type molecules and in particular of a- and p-boswellic acids, and their O-acetates [104 111], 11 -Oxo-p-boswellic acid and its acetyl derivative, identified in several Boswellia species, are also diagnostic for frankincense [112]. 

The survival of a-boswellic acid, p-boswellic acid and their O-acetates, which have been isolated only from frankincense, has been demonstrated in archaeological samples [99,107,113]. These compounds are considered as very useful specific chemical markers for the identification of frankincense in resinous archaeological materials. 

Figure 8.3 Mass spectra of (a) a boswellic acid, (b) (3 boswellic acid and (c,d) their O acetates in the form ofTMS derivatives... 

The [4+2] cycloaddition of dimethyl-1,2,4,5-tetrazine-3,6-dicarboxylate 41 with ketene A, O-acetals or cyanamide yielded tetrafunctionalized pyridazines 42 or 1,2,4-triazine 43 respectively. Treatment of 42-43 with zinc dust in AcOH afforded pyrrole 44 or imidazole 45 derivatives <06S1513>. 

Pd-catalyzed intramolecular etherification reactions of aliphatic alcohols have also been practiced in tandem with other bond-forming processes, such as a Pd-catalyzed allyltin addition to an aldehyde (Equation (32)).160 Similarly, a tandem C-N and G-O bond formation sequence occurs (Equation (33)) during the reactions of /3-amino alcohols with biscarbonates in the presence of the N,O-acetal-derived ligand 43.161-163... 

Hydroformylation in the presence of nucleophiles such as alcohols or amines leads to 0,0- or N,O-acetals, aminals, imines or enamines (Scheme 2). 

Hydroformylation of unsaturated amines offer a convenient synthetic access to cyclic AT.O-hemiacetals. If performed in the presence of alcohols or orthoesters AT,O-acetals are formed. With additional N-nucleophiles N,N-acetals are obtained. These compounds are synthetically attractive building blocks and were therefore used as a key step in the synthesis of various natural products [27,35]. Thus the synthesis of (+)-prosopinine starting from enantiopure (T)-scrinc leads to a cyclic N,O-acetal functionality with the required functionality for the attachment of the side chain (Scheme 6) [36]. 

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