Subject synthetic equivalents

This particular feature of P-N bond containing compounds can be utilized for synthetic purposes by using "phosphoryl protection" in the synthesis of amines based on alkylative procedures. The solution of this problem introducing two new reagents, i.e. diphenyl-phosphinic amide (I) and sodium N-(t-butyloxycarbonyl) diethyl phosphoroamidate (II) as useful synthetic equivalents of an amino moiety is the subject of this communication. 

The synthetic equivalency of the silane and an alcohol can be easily demonstrated by subjecting the crude silane product to any of a variety of available oxidizing conditions (Eq.48) [30,40]. 

Nucleophilic ring opening of the lactone ring in bicycloadducts of type 5 leads directly to tetrasubstituted cyclohexenes in which the relative stereochemistry of all four contiguous stereocenters is established. Thus, pyrones provide attractive synthetic equivalents to acyclic dienes of type 6 which may be difficult to prepare as pure geometrical isomers and which in many cases do not lead via Diels-Alder cycloaddition to the desired stereochemical relationships. The application of [4+2] cycloaddition reactions of 2-pyrones to synthesizing functionalized cyclohexenes was the partial subject of a 1994 review. ... 

When furan or substituted furans were subjected to the classic oxidative coupling conditions [Pd(OAc)2 in refluxing HOAc], 2,2 -bifuran was the major product, whereas 2,3 -bifuran was a minor product [12,13]. Similar results were observed for the arylation of furans using Pd(OAc)2 [14]. The oxidative couplings of furan or benzo[i]furan with olefins also suffered from inefficiency [15]. These reactions consume at least one equivalent of palladium acetate, and therefore have limited synthetic utility. 

The synthetic utility of a-phosphorus- and a-thio-stabilized carbanions is the subject of numerous reviews.21 Notable are additions of phosphonium ylides (237),183 sulfonium ylides (238),l84 ° oxosulfo-nium ylides (239)184 " and sulfoximine ylides (240)184,1 to electron-deficient alkenes which afford nucleophilic cyclopropanation products. In contrast, with a-(phenylthio)-stabilized carbanions, which are not acyl anion equivalents, either nucleophilic cyclopropanation or retention of the hetero substituent occurs, depending on the acceptor and reaction conditions used. For example, carbanion (241) adds to 1,1-... 

However, spectroscopic studies of activated BLM indicate that it is not an Fev=0 species. It exhibits an S - 1/2 EPR spectrum with g values at 2.26, 2.17, and 1.94 [15], which is typical of a low-spin Fe111 center. This low-spin Fem designation is corroborated by Mossbauer and x-ray absorption spectroscopy [16,19], Furthermore, EXAFS studies on activated BLM show no evidence for a short Fe—0 distance, which would be expected for an iron-oxo moiety [19], These spectroscopic results suggest that activated BLM is a low-spin iron(III) peroxide complex, so the two oxidizing equivalents needed for the oxidation chemistry would be localized on the dioxygen moiety, instead of on the metal center. This Fe(III)BLM—OOH formulation has been recently confirmed by electrospray ionization mass spectrometry [20] and is supported by the characterization of related synthetic low-spin iron(III) peroxide species, e.g., [Fe(pma)02]+ [21] and [Fe(N4py)OOH]2+ [22], The question then arises whether the peroxide intermediate is itself the oxidant in these reactions or the precursor to a short-lived iron-oxo species that effects the cytochrome P-450-like transformations. This remains an open question and the subject of continuing interest. 

The Aspidosperma family of indole alkaloids has inspired many synthetic strategies for the construction of their pentacyclic framework of the parent compound aspidospermidine (366), since the initial clinical success of two derivatives, vinblastine (10) and vincristine, as anticancer agents. The alkaloids such as (-)-rhazinal (369) and (-)-rhazinilam (6) have been identified as novel leads for the development of new generation anticancer agents [10,11]. Bis-lactams (-)-leucunolam (370) and (-t-)-epi-leucunolam (371) have bio-genetic and structural relationships with these compounds [236]. Recently, enantioselective or racemic total syntheses of some of the these natural product were achieved. One successful synthesis was the preparation of the tricyclic ketone 365, an advanced intermediate in the synthesis of aspidospermidine (366), from pyrrole (1) (Scheme 76) [14]. The key step is the construction of the indolizidine 360, which represents the first example of the equivalent intramolecular Michael addition process [14,237,238]. The DIBAL-H mediated reduction product was subject to mesylation under the Crossland-... 

Pive-membeFed ring systems are rapidly emerging as important structural features in a large number of natural products and theoretically interesting molecules. The development of methodologies for the construction of these systems has Aus become a subject of great interest and intense effort for synthetic chemists. In recent years, one has seen a number of C5 annulation procedures, with the majority focusing on multistep sequences based on 1,4-dicarbonyl compounds or their functional equivalents. 

A novel synthetic approach was developed by R.E. Taylor et al. for the preparation of the triene portion of the biologically active polyketide apoptolidin. The allylic chloride substrate was prepared from an allylic alcohol via a thionyl chloride mediated rearrangement. Next, the allylic chloride was subjected to the Ganem oxidation by treating it with five equivalents of trimethylamine A/-oxide (TMANO) in DMSO at room temperature to obtain the desired a,p-unsaturated aldehyde. Interestingly, the original Kornblum oxidation conditions were not well suited for this system because of the required high reaction temperature. 

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