Oxaphospholens

2-Oxaphospholens.—Among new ajS-unsaturated ketones used in the formation of 1 1 adducts with tervalent phosphorus compounds are dibenzylidenecyclohexanone, ethyl benzylidenebenzoylacetate, and the allenic ketones (51). Hydrogen chloride and the trimethyl phosphite adduct (52) gave the conjugated (53) and non-conjugated (54) isomers in 

The equilibration and the variable-temperature and n.m.r. spectra of the isomeric adducts formed from benzylideneacetylacetone and the ethylene phosphites (56 R = OMe or NMea) have been studied. In 

The thermally stable compound, m.p. 170—172°C, formed from the ylide (57) and p-bromobenzaldehyde, has been shown by A -ray analysis to be the dioxyphosphorane (58). 

2-oxaphospholens from trimethyl phosphite and but-3-en-2-one (44) and benzylideneacetylacetone (45) are powerful D-alkylating agents, converting acids and phenols into esters and ethers, respectively. Mesiloic acid is esterified at room temperature. The mechanism is probably as shown in (46). 

2- Oxazaphos olans.—About forty new spirophosphoranes containing a 

2- oxaza-phospholan or -phospholen ring and a P—H bond have been prepared and their equilibria with the corresponding tervalent phosphorus species studied. Symmetry is an important factor in determining the position of these equilibria. Further examples have been described of second-order asymmetric induction in the crystallization of spirophosphoranes derived 

Spirophosphoranes (50) containing a 4-keto-l,3,2-oxazaphospholan ring have been prepared as shown. In other cases, e.g. (51) and (52), only the tervalent form was obtained. 

---

Dichloroethylphosphine has been shown to react with methyl vinyl ketone to form 2-ethyl-5-methyl-A -l,2-oxaphospholen-2-oxide (25), which has been converted to (26) by chlorination in the presence of base. The same phosphine adds to methyl acrylate in the presence of acetic acid to give the phosphine oxide (27). Further examples have appeared of the reactions of the phenylhydrazones of methyl ketones with phosphorus trichloride to produce the heterocycles (28). 

R =Pr R =H). Finally, terminally-disubstituted compounds yield 1,2-oxaphospholenes (175) exclusively. 

Preparation of 2,2,2-trimethoxy-3-phenyl-4-acetyl-5-methyl-A4-oxaphospholene — Preparation of an oxyphosphorane by reaction of a trialkyl phosphite with an a,(l-unsaturated carbonyl compound... 

Preparation of 2,2,2-triethoxy-2,2-dihydro-5-methyl-l,2A,5-oxaphospholene —Preparation of an oxyphosphorane... 

Trimethyl phosphite (11.3 g, 0.091 mol) was added to a solution of 3-benzylidene-2,4-pentanedione (16.35 g, 0.091 mol) in dry methylene chloride. The solution was maintained under nitrogen for 24 h at 20°C and for an additional 5 h at 40°C. After this time, the solvent was evaporated, and the residue was dissolved in hexane. These actions were performed in the absence of moisture. The clear hexane solution was seeded with a crystal of the pure crystalline product (obtained by crystallization from hexane by standing for 2 weeks at 0°C), and after 8 h at 0°C the crystals precipitated yielding pure 2,2,2-tri-methoxy-3-phenyl-4-acetyl-5-methyl-A4-oxaphospholene (25.12 g, 88.4%) of mp 48-51°C. 

Preparation of diethyl [(2R )-2- l(S )-hy droxyphenyl -3-oxobutyl]phosphonate — Preparation of a functionalized phosphonate by reaction of an oxaphospholene with an aldehyde35... 

McClure, C.K., Grote, C.W., and Rheingold, A.L., Novel and efficient synthesis of uracil phosphonate derivatives via pentacovalent oxaphospholenes, Tetrahedron Lett., 34, 983, 1993. 

In 1972, Buono et al. observed that tricoordinated phosphorus compounds RP(OMe)2 (R = Me, Ph, CH=CH2, SMe, CN, OPh, NMe2) or Ph2POMe reacted with 1,2-propadienyl ketone 446 to afford 3-alkylidene-A4-l,2-oxaphospholenes 447 [201, 202],... 

In exploring various synthetic methods for the preparation of 1,2-oxaphospholene 2-oxides we investigated the reaction of diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) with methyl and phenylphosphonous dichloride, which was reported (1) to proceed as shown in eq (1) to give 2,3,3,5-tetramethyl-l,2-oxaphos-phol-4-ene 2-oxide or the corresponding 2-phenyl derivative. 

Interesting derivatives of indole were described in [35]. When 3-ethoxy carbonyl-2-methyM-(trimethylammonium)methyl-5-hydroxyindole iodide [162, 163] is heated with an excess of trialkyl phosphite [64] (160-165°C, 30 min) 3-ethoxycarbonyl-2-methyl-4,5-(2-oxo-2-alkoxy-l,2-oxaphospholene-4,5)indoles 65... 

Fig. 2. Molecular structure of a fused 1,2-oxaphospholene-4 and 1,2,5-dioxaphot-phorinene-3 ring system with pentavalent phosphorus, from X-ray crystallographic analysis.

Scheme 3 contains several of the fundamental structures with penta-valent phosphorus, among them phosphorane itself, PH6 (21), and pentahydroxyphosphorane, P(OH)6 (26). The latter is the hydrate of phosphoric acid H3P04+H20 - P(OH)5. A hydrate of methylphos-phonic acid (CH3)(HO)4P (29), is also included as a model for its esters. Mono- and di-ionized forms, (27), (28), (30), are also given, since their stabilities in various isomeric forms provide important data concerning the role of intermediate oxyphosphoranes in the chemistry of phosphoric acid and its derivatives. A model compound of the 1,2-oxaphospholene ring (31), is provided, since this system, in the form of several derivatives, will be discussed extensively in Section VIII. 

The two tetraoxyphosphoran.es (82) and (83) in Scheme 10 have in common a cis relationship of two identical substituents on the five-membered ring. There are some similarities between (82), (83), and the derivative of the 1,2-oxaphospholene (70) of Scheme 8, since in both cases, the five-membered rings (including its carbon-substituents) have... 

The TBP-phosphorus atom with three equivalent ligands can no longer be a chiral center. As a consequence, the positional exchange of ligands in the 1,2-oxaphospholene (64) (Scheme 15) reported a few years... 

Oxidation of Oxaphospholenes. Reaction of oxaphos-pholene (5) with (+)-[(8,8-dichlorocamphoryl)sulfonyl] oxaziri-dine (2) affords p-hydroxy-y-keto-phosphonate in 49% ee with undetermined absolute configuration (eq 3). Higher temperatures accelerate the reaction but lower the stereoselectivity. 

In an elegant extension of their pentacoordinate oxaphospholene methodology which uses the phosphorane as an enolate equivalent, McClure and Mishra synthesized (39) from (38) and triethyl phosphite and then proceeded to show that (39) could be converted to (40) a potential precursor to the biologically important sphingosine-1-phosphate (41) ... 

Triethoxy-l,2A5-oxaphospholene (112) reacts at room temperature with 2-(p-tolyl)-3-phenyl-oxaziridine (63d) to give a /Miydroxy-y-ketophosphate (113) in 82% yield (Equation (26)) (91TL5313). It seems likely that an oxirane intermediate is initially formed which rearranges on work-up to (113). Asymmetric oxidation with (+ )-[(8,8-dichlorocamphoryl)sulfonyl]oxaziridine (74) gave (113) in 49% ee and 88% isolated yield. The absolute configuration of (113) was not established. 

Treatment of the A -l,2-oxaphospholen (100 R ==OMe) with LiBr in MeOH yields the salt (101) together with the open-chain compounds (102) and (103) in the proportions 1 1 2. The formation of (102), and particularly the high yield of (103), has been advanced as good evidence for the participation of the intermediate (104), from which transfer of axial methyl group to Br occurs. ... 

The reaction of trimethyl phosphite with an allenic chloromethyl ketone proceeds with the formation of an oxaphospholene and not through the Michaelis-Arbuzov or Perkow rearrangements. ... 

A related route to 3-oxoalkylphosphonates involves the conjugate addition of trialkyl phosphites to a,P-unsaturated ketones and hydrolysis of the intermediate oxaphospholenes. The utility of this method is enhanced by the nucleophilic character of the intermediate oxaphosphorane, which facilitates the stereoselective aldol reaction resulting in the formation of p-substituted y-ketophos-phonates. - The pentacovalent oxaphospholene reacts with dialkyl azidocarboxylates to give P-hydrazido-y-ketophosphonates in excellent yields. ... 

---