Adam
Throup
,
Laurence H.
Patterson
and
Helen M.
Sheldrake
*
Institute of Cancer Therapeutics, University of Bradford, Bradford, BD7 1DP, UK. E-mail: h.sheldrake@bradford.ac.uk
First published on 20th September 2016
Fused cyclobutanes are found in a range of natural products and formation of these motifs in a straightforward and easy manner represents an interesting synthetic challenge. To this end we investigated an intramolecular variant of the thermal enamine [2 + 2] cyclisation, developing a diastereoselective intramolecular enamine [2 + 2] cyclisation furnishing δ lactone and lactam fused cyclobutenes in good yield and excellent diastereoselectivity.
Intramolecular variants of the photochemical [2 + 2] cyclisation have been developed and used in synthesis, but enamine [2 + 2] cyclisation has been underexploited to date and its full scope has not been explored. Intramolecular cycloaddition of ketenes or ketene-iminium salts with alkenes provides a useful route to [n.2.0] fused-cyclobutanones,14–18 however ketene cycloadditions provide a limited pattern of substituents on the cyclobutane ring and do not allow easy access to cyclobutenes. The development of an intramolecular enamine [2 + 2] cyclisation gives a complementary albeit contrasting route to functionalised fused cyclobutanes and cyclobutenes. The recent identification of the cyclobutene moiety as a suitable electrophile for targeted covalent modification of proteins19 suggests this functionality will have increasing importance in medicinal chemistry. The development of straightforward procedures for the synthesis of 4-membered rings leading to predictable product structures is therefore an important challenge.
Only the 6,4 fused lactones 9b and 9e were isolated. The other length tethers failed to cyclise; in each case most of the aldehyde was consumed but the desired compounds (9a, 9c, 9d) were not isolable and the products 9 or corresponding intermediates (8a, 8c, 8d) were not observable by 1H NMR of the crude reaction mixtures. This indicated failure of the cyclisation step.
Cyclisation of 7a to form a 5,4 fused bicycle was probably prevented by the tether being too short to allow a favourable orientation of orbital overlap as the two reacting group approach one another; a conformation where the two halves of the molecule are close enough to allow cyclisation would place the π-system of the fumarate and aldehyde-derived enamine to be perpendicular to one another. Furthermore, Ghosh et al.20 have shown that a similar ester tethered system did not undergo photochemical cyclisation since the preferred conformation (7a S-cisFig. 2) holds the two ends of the chain at a distance from one another. Longer tethers (7c,d) will be affected by the entropic issues well-known for reducing efficiency of macrocycle formation. In this case, even the S-trans configuration does not bring the two ends of the molecule into sufficiently close proximity to allow a reaction. Small amounts of starting aldehyde were recovered from some reactions, along with complex mixtures of products also containing aldehyde groups (starting material was generally not recovered from successful reactions).
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Fig. 2 Non-reacting (S-cis) and reacting (S-trans) conformations of ester tethered cyclisation substrates. |
Attempts to optimise the reaction conditions were limited (Table 2) as it has already been established that this type of reaction tolerates a limited range of reagents and conditions; acetonitrile is required to stabilise the zwitterionic intermediates,12,13 (we have found that related reactions attempted in THF failed) and the presence of K2CO3 is required for cyclobutane formation.13 Decreasing the concentration of the reaction in an attempt to prevent polymerisation did not improve the yield of isolable cyclobutene products, which suggests polymerisation is not the major side reaction taking place. 7a may decompose in the presence of base by elimination of the fumarate group to yield acrolein. Michael addition of diethylamine to the starting material would reversibly prevent productive reaction of any substrate. Diethylamine was the only amine investigated which gave a successful reaction. Cyclic amines (entries 13–15) caused complete decomposition of the reaction mixture; (R)-N,α-dimethylbenzylamine did not cause cyclobutane formation, or react with the fumarate group in 7b. Chiral amine auxiliaries cannot be used to generate homochiral cyclobutanes by this method.
Entry | Precursor | Concentration | Amine | Yield |
---|---|---|---|---|
a 10% aldehyde recovered. b 16% aldehyde recovered. c 35% aldehyde recovered. | ||||
1 | 7a | 0.045 M | Et2NH (1 eq.) | 0 |
2 | 7a | 0.045 M | Et2NH (2 eq.) | 0 |
3 | 7b | 0.050 M | Et2NH (1 eq.) | 29 (9b) |
4 | 7b | 0.051 M | Et2NH (2 eq.) | 37 (9b) |
5 | 7b | 0.066 M | Et2NH (2 eq.) | 43 (9b) |
6 | 7b | 0.075 M | Et2NH (2 eq.) | 41 (9b) |
7 | 7b | 0.034 M | Et2NH (10 eq.) | 28 (9b) |
8 | 7e | 0.045 M | Et2NH (2 eq.) | 20 (9e) |
9 | 7e | 0.097 M | Et2NH (2 eq.) | 16 (9e) |
10 | 7c | 0.050 M | Et2NH (2 eq.) | 0a |
11 | 7d | 0.032 M | Et2NH (2 eq.) | 0b |
12 | 7d | 0.024 M | Et2NH (2 eq.) | 0c |
13 | 7b | 0.050 M | Pyrrolidine (2 eq.) | 0 |
14 | 7b | 0.050 M | Morpholine (2 eq.) | 0 |
15 | 7b | 0.080 M | (R)-2-Methylpyrrolidine (2 eq.) | 0 |
16 | 7b | 0.080 M | (R)-N,α-Dimethylbenzylamine (2 eq.) | 0 |
Increasing the steric bulk in the substrate by switching from ethyl to tbutyl ester (7e) resulted in reduced yield of 9e after increased reaction time (5 days compared to the standard 2 days for intramolecular reaction of ethyl esters).
After the successful preparation of the lactone fused cyclobutene, synthesis of lactam fused cyclobutenes was attempted. Lactam 6,4 fused cyclobutanes have previously been accessed by photochemistry,21–23 but the majority of work involves quinolone-based structures. Lactam 5,4 fused cyclobutanes are accessible through a wider range of methods, frequently relying on prior preparation of a 1,2-cyclobutane dicarboxylic acid or photocyclisation of a maleimide.24–27 Here, amide tethered aldehydes were synthesised containing a Boc protected 7f, and benzyl protected 7g amide (the corresponding unprotected aldehyde was not accessible due to side reactions during oxidation). When subjected to the cyclisation conditions both the Boc and benzyl protected amides cyclised forming lactam fused cyclobutenes 9f,g, although in lower yields than their ester linked counterparts. The method therefore complements photochemical cyclisations, which tolerate the presence of an amide NH group.
Cyclisation substrates 12a–e were prepared from a range of alcohols 10a–d and amine 10e by coupling with ethyl hydrogen fumarate, deprotection and oxidation to afford the required aldehydes. Cyclisation/eliminations proceeded in good to moderate yield over the 3 steps; all reactions yielded a single diastereomer (>95% de) (Table 3). This was shown to be the all cis compound by nOe spectroscopy; crosspeaks were observed between H-1 and H-6 indicating the expected cis-configuration at the ring junction, and between H-4 and H-6/H-1 indicating H-4 is on the same face of the molecule as the protons at the ring junction (e.g.Fig. 3).
The excellent diastereoselectivity, and the cis arrangement of groups around the 6-membered ring, is hypothesised to arise through a chair-like transition state 14 (Fig. 4) where the side chain ‘R’ in a pseudoequatorial position is favoured. This conformation allows efficient overlap of the two π systems to facilitate cyclisation. The alternative conformation 15 places the π systems near perpendicular to one another, making efficient overlap to form a cis-fused bicycle not possible.
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Fig. 4 Proposed transition states for intramolecular cycloaddition. Pseudoequatorial arrangement of all sidechains during cyclisation leads to the observed product 13. |
The yields of cyclobutenes 13 were generally higher than the yields of cyclobutenes 9 obtained from cyclising unsubstituted precursors, presumably due to the presence of the sidechain reducing the conformational flexibility of the substrate and favouring a reactive conformation. However, the increased yield did not generalise to the formation of larger ring systems (Fig. 5). Compound 19 (an isomer of 13d) was of interest since both 19 and 13b are suitable starting points for synthesis of the highly substituted cyclobutane ring system found in providencin 5. Compound 21, which is similar to known intermediates such as 20,28 could be obtained from either 19 or 13b by rhodium-catalysed addition29 of furan to the cyclobutene and further elaboration of the sidechains. Subjecting aldehyde 18 to the cyclisation conditions for up to 7 days returned unreacted starting material along with small amounts of decomposition products resulting from aldol condensations of 18.
In conclusion, we have demonstrated that an intramolecular variant of the enamine [2 + 2] cyclisation allows straightforward access to 6,4 fused bicyclic systems in a diastereoselective manner. This methodology provides a new means to access functionalized fused cyclobutenes bearing sidechains that allow further synthetic elaboration. Applications of the methodology in drug discovery, and to the total synthesis of the biologically active natural product providencin, are in progress.
This work was funded by Yorkshire Cancer Research (Award reference number B002-PhD). We thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for measurement of HRMS data.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ob01661h |
This journal is © The Royal Society of Chemistry 2016 |