Toward accessible 3 H -pyrroles

Yet recently difficult-to-access 3 H -pyrroles, rare nonaromatic pyrrole systems, now have become readily available due to essential modification of their synthesis from aryl isoalkyl ketoximes and acetylene in the KOH/DMSO system allowing to carry out the process under atmospheric pressure. In contrast to the known procedure implemented under pressure, the modified method is expedient, much safer and feasible that makes it friendly for any laboratory.

2][13][14] Just recently, a method, which can be rated as having some generality, has been published. 10This method is based on the reaction between ketoximes with only one C-H bond neighboring to the oxime function and acetylene in the presence of KOH/DMSO system.Although the advantages of this protocol are obvious, it suffers from using the compressed acetylene (10-13 atm) that requires special facilities, safety precautions and expertise for its implementation.All this impedes a broader application of such a synthesis and hence 3Н-pyrroles keep remaining difficult-to-access.
Therefore, the purpose of the present work was to improve the above synthesis using pressurized acetylene to make it safer and more practically feasible so that it could be rendered in any laboratory without special facilities.Generally, it is clear enough that in the acetylene chemistry the transfer from pressure synthesis to that under atmospheric pressure is not always achievable and hence, when done, represents a remarkable novelty since it greatly enhances the safety and feasibility of the method.
Until now the systematic investigations of this reaction in a flow reactor avoiding the use of compressed acetylene were not carried out.Particularly, it was unclear whether this pressure-free procedure could replace the method based on the use of compressed acetylene preserving the same scope of the substituents.
Commonly, [15][16][17] the synthesis of pyrroles from ketoximes and acetylene is considered as a cascade reaction starting from the formation of O-vinyl ketoxime A, which undergoes 3,3-sigmatropic shift to iminoaldehyde B further cyclizing to hydroxypyrroline C and dehydration to the final product (Scheme 1).
Consequently, the key step of the synthesis is the intramolecular regioselective C-H functionalization of the C-H bond adjacent to the oxime function readily transforming it into an aldehyde group.Here it should be noted that mild and selective C-H functionalizations are justifiably claimed as a mainstream of organic chemistry. 18n the case of aryl(hetaryl) isoalkyl(cycloalkyl) ketoximes, the pyrrole assembly stops at the stage of formation of 3,3-dialkylsubstituted 3H-pyrroles, which cannot isomerize to aromatic 1Hpyrroles (Scheme 2).
Recently, 10 it was proved that for transformation of hydroxypyrrolines to 3H-pyrroles, vinylation of the former under acetylene pressure was required and the final step of the assembly was superbase-promoted elimination of vinyl alcohol (acetaldehyde) from O-vinyloxypyrrolines

Results and Discussion
For the optimization of 3H-pyrrole synthesis under atmospheric pressure, we have chosen isopropyl phenyl ketoxime 1a.The reaction was performed under atmospheric pressure (flow reactor) in the MOH (M = Na, K, Cs)/DMSO superbase system.Acetylene was passed through the stirred reaction mixture with a rate of 40-45 mL/min (12.5 mmol of ketoxime 1a, 25-100 mL of DMSO).The following parameters were varied: type of the superbase system, ketoxime : base molar ratio, co-solvent (n-octane), the reactants concentration, temperature and time.
As follows from Table 1, the conversion of ketoxime 1a ranged 75-96%, while the yields of pyrrole 2a spanned 20-33%.The best result (33% isolated yield of 2a at 90% conversion of 1a) was attained in superbase system KOH/DMSO (1a : KOH molar ratio = 1 : 1, Entry 2, Table 1).In the NaOH/DMSO system, the yield of pyrrole 2a is much lower (20%, Entry 1).In this case, the reaction mixture contained 31% of hydroxypyrroline 3а, which was isolated in 11% yield (Scheme 4).At either lower (0.5 equiv.)or higher (1.5 equiv.)content of KOH in the reaction mixture (Entries 5 and 6), the yield of 2a changed insignificantly (32 and 30%, respectively).However, at the equimolar ratio of 1a : KOH (Entry 2), the number and quantities of side products decreased that makes easier isolation of pyrrole 2a.The 75% conversion of the ketoxime 1a was reached already after 2 h, the yield of pyrrole 2a being 29% (Entry 7).When the reaction time increased up to 6 h (Entry 8), conversion of 1a became almost complete (96%), while the yield of pyrrole 2a slightly dropped (28%).At both lower (25 mL of DMSO, Entry 9) and higher (100 mL of DMSO, Entry 10) dilution vs. optimal one (12.5 mmol of 1a in 50 mL of DMSO), the yield of 2a noticeably diminished (23 and 22%, respectively).In the synthesis of 3H-pyrroles under pressure, 10 the non-polar co-solvent (n-hexane) allowed to considerably increase (by about 10%) the yields of the target products.However, in this case, the application of a similar mixed solvent (50 mL of DMSO and 40 mL of n-octane) led to the decreased yield of pyrrole 2a (29%, Entry 11).Thus, this pressure-free protocol happened to be even simpler (with regard to solvent) then the synthesis under pressure.
Noteworthy, the isolated yields of 3H-pyrroles are about twice as less compared with the content of the target pyrroles in the crude product.It might be due to further transformations of the products on chromatography column, e.g.silica-catalyzed addition of unreacted ketoximes to the double bond of 3H-pyrroles.At the same time, partial hydration of 3H-pyrrole 2a early observed 10 when passing through chromatography column on aluminium oxide has not occurred on silica (neat 3H-pyrrole 2a lost just about 2 wt %).
Taking into account such a high sensitivity of 3H-pyrroles to the isolation protocols, it might be understandable the failure to reproduce the yield of 3H-pyrrole 2a reported in 1991 (53%) 19 that may be caused by the differences in quality and nature of the sorbents and eluents employed.In fact, the content of this pyrrole in the crude product (Table 1) is 64%.
The only exception was 2-(2,5-dimethylphenyl)-3,3-dimethyl-3H-pyrrole 2e formed under the above conditions just in 8% yield (Scheme 5), while the major product was 5-vinyloxypyrroline 4e, which was identified in the reaction mixture by its 1 H NMR spectrum.As mentioned above (Scheme 3), this compound is formed by vinylation of the corresponding intermediate hydroxypyrroline.3H-Pyrrole 2e was obtained (21% yield) by additional treatment of the reaction mixture with 3 equiv.of KOBu t at 120 °C for 30 min, during which the intermediate 5-vinyloxypyrroline 4e was transformed into the target product (Scheme 5).

Conclusions
An expedient, pressure-free, safe and feasible synthesis of 3H-pyrroles, until now difficult-toaccess, has been developed improving the cascade reaction of ketoximes, having only one C-H bond adjacent to the oxime function, with acetylene.The synthesis does not require transition metal catalysts, utilizes inexpensive starting materials and can be implemented using ordinary laboratory facilities.Consequently, 3H-pyrroles have been rendered from yet recently exotic compounds to readily available family of promising potent synthetic intermediates for the preparation of biologically active compounds and reactive building blocks for organic synthesis.

Table 2 .a
Synthesis of 3H-pyrroles 2a-f from ketoximes 1a-f and acetylene a Conditions: oxime 1 (12.5 mmol), KOH (12.5 mmol), DMSO (50 mL), acetylene (atmospheric pressure), 90 °C.b According to 1 H NMR data of the crude product.c The yield was calculated on the reacted ketoxime.d The initial reaction mixture was additionally treated with KOBu t (3.0 equiv.) at 120 °C for 30 min, see text below.e Too complex 1 H NMR spectrum of the crude product.