Pyrrolophanes

H,3H- Pyrrolo[l, 2-c]oxazole-l, 3-dione, 5,6,7,8-tetrahydro-IR spectra, 6, 978 [2.2](2,5)Pyrrolophane, N-aryl-rearrangements, 4, 209 Pyrrolophanes natural products, 7, 764 synthesis, 7, 771 Pyrrolophanes, N-aryl-synthesis, 7, 774 (2,4)Pyrrolophanes synthesis, 7, 771 Pyrrolo[3,4-c]pyran-4-ones synthesis, 4, 288 Pyrrolopyrans synthesis, 4, 525, 526 Pyrrolopyrazines synthesis, 4, 526 Pyrrolo[l, 2-a]pyrazines synthesis, 4, 516 Pyrrolo[2,3-6]pyrazines Mannich reaction, 4, 504 Vilsmeier reaction, 4, 505 Pyrrolo[3,4-c]pyrazole, 1,3a,6,6a-tetrahydro-structure, 6, 976 synthesis, 6, 1019 Pyrrolopyrazoles synthesis, 5, 164 Pyrrolo[l,2-6]pyrazoles synthesis, 6, 1002, 1006 Pyrrolo[3,4-c]pyrazoles reactions, 6, 1034 synthesis, 6, 989, 1043 Pyrrolo[3,4-c]pyrazolones synthesis, 6, 989 Pyrfolopyridazines synthesis, 4, 517 Pyrrolo[l, 2-6]pyridazines synthesis, 4, 297 6/7-Pyrrolo[2,3-d]pyridazines synthesis, 4, 291 2/f-Pyrrolo[3,4-d]pyridazines synthesis, 4, 291 6/7-Pyrrolo[3,4-d]pyridazines synthesis, 4, 291... 

N,N, 1,1,2,2,8,8,9,9-Decamethyl-1,2,8,9-tetrasila[2.2](2,5)pyrrolo-phane (30) is prepared by a procedure similar to that applied to 28 and 29 through l,2-bis(A(-methylpyrrolyl)disilane 31 with slight modification of the reagents. The lithiation was performed with f-BuLi/ TMED. Under dilute conditions pyrrolophane 30 was obtained in 4% yield from 31 as air-stable colorless crystals besides a polymeric residue (35) (Scheme 7). 

H NMR of pyrrolophane 30 shows two singlets for SiMe groups that do not coalesce in the temperature range of -80 to +110°C, in contrast to furanophane 28, which shows rapid anti/syn isomerization down to -78°C, and thiophenophane 29, with a coalescence temperature of 15°C. The rigidity of 30 is obviously due to the steric bulk of the 2V-methyl groups. 

The great steric demand of the methyl group on N was further demonstrated by NMR characteristics of Si-O-Si bridged pyrrolophane 32 prepared by oxidation of 30 with excess Me3N-0 in refluxing benzene (74% yield). Even with the considerably longer Si-O-Si bridges, no anti/syn isomerization occurs for 32 between 80 and +110°C. 

Insertion of oxygen into the Si-Si bond of 30 also blocks ct—it conjugation effectively. The fact can be seen by the shift toward short wavelength for both absorption of 31 in UV (Amax = 239 nm) (Fig. 16) and the CT complex with TONE (Amax 593 nm) as compared to pyrrolophane 30 (Fig. 17). 

An unusual and novel rearrangement of the Af-aryl[2.2](2,5)-pyrrolophane (15) has been induced by treatment with acetic acid at 100 °C (79JOC2498). The proposed mechanism, shown in Scheme 11, has no supporting evidence. 

An interesting application of PET mediated bond cleavage reaction from azirine 63 has been reported by Mattay et al. [67] for synthesizing N-substituted imidazoles (65) via the (3 + 2) cycloaddition reaction of resultant 2-azaallenyl radical cations with imines 64. Synthesis of pyrrolophane 3,4-dimethyl ester (68) has been reported recently [68] by the ring opening of 66 followed by inter-molecular cycloaddition with dimethyl acetylene dicarboxylate (67) as shown in Scheme 13. 

A platinum- and Lewis acid catalyzed enyne metathesis was used as the key step in the formal total synthesis of antibiotics streptorubin B and metacycloprodigiosin by A. Furstner. The electron-deficient enyne was cyclized with either a platinum halide or a hard Lewis acid (e.g., BF3-OEt2) to the desired mefa-pyrrolophane core of the target molecules. A few more steps completed the formal synthesis. 

In the laboratory of A. Ftirstner, a practical synthesis of the immunosuppressive aikaioid metacycioprodigiosin and its functional derivatives was developed. Toward the end of the synthetic sequence a mefa-pyrrolophane (3-keto ester was decarboxylated under standard Krapcho conditions. The substrate was dissolved in wet DMSO, and two equivalents of sodium chloride were added and the reaction mixture was heated to 180 °C to afford the desired meta-pyrrolophane ketone in excellent yield. This ketone functionality was first converted to an ethyl group and then the product was advanced to metacycioprodigiosin. 

Another important application of DCN-sensitized photodissociation of strained ring compoimds has been demonstrated by Muller and Mattay [127] for synthesizing iV-substimted imidazoles (147) by the [3-l-2]-cycloaddition of the 2-azaallenyl radical cation (144), produced by the cleavage of corresponding radical cation from azirine (143), with imines. This strategy is further extended [128] for the synthesis of pyrrolophane 3,4-dimethyl ester (152) by the ring opening cycloaddition reaction of (148) with dimethyl acetylene dicarb-oxylate (Scheme 32). 

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