Rapid synthesis of 1,7-bis(t-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (DO2A-t-Bu ester)

Abstract

A three-step route was used to synthesize 1,7-bis(t-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (DO2A-t-Bu ester) from 1,4,7,10-tetraazacyclododecane (cyclen). The overall time of reaction was reduced from a combined ~56 h to 2.3 h with an overall yield comparable to previously reported methods.

Derivatives of 1,4,7,10-tetraazacyclododecane (cyclen), including 1,7-bis(t-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (DO2A-t-Bu ester, 1), are commonly used intermediates when synthesizing metal complexes for biomedical-related studies.^1–19 However, reported routes to synthesize and purify this intermediate require multiple days to complete.^20–22 We hypothesized that the reactions used to produce 1 could be accelerated by modification of the reaction conditions, leading to a shortened overall synthetic time while maintaining comparable yields to previously reported reactions.

The reported synthesis of DO2A-t-Bu ester, 1, is carried out in three steps as depicted in Scheme 1.^20–22 First, two amines of commercially available cyclen are protected in the trans positions with a slow addition of CBZCl at 0 °C and stirring overnight.²⁰ Second, t-Bu bromoacetate is reacted with CBZ-protected cyclen, 3, at 60 °C in the presence of diisopropylethylamine (DIEA) to produce macrocycle 4 in 20 h.²¹ Finally, hydrogenation for 1 d removes the CBZ groups yielding the desired product 1.²¹

Scheme 1.

Reported synthesis of DO2A-t-Bu ester, 1, with shortest reported times shown.^20–22

We observed that the reaction time from 2 to 3 can be reduced with the application of heat, leading to a decrease in reaction time from 12 h to 30 min at reflux (Scheme 2). Temperatures below reflux led to either longer reaction times or lower yields. We verified that heating did not produce the undesired cis-substituted product by comparing our final product, 1, with a reported NMR spectrum of 1,4-bis(t-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane.²³ Based upon our observations, heating the first step at reflux leads to the exclusive formation of the trans-substituted intermediate 3.

Scheme 2.

Modified synthesis of 1 with modifications shown in red.

The reported reaction of 3 to 4 requires heating at 60 °C with 2 equiv of base for 20 h to reach completion. We predicted that the addition of excess base would allow for a shorter overall reaction time because of the similar pK_a values of DIEA and the amines on macrocycle 3 (all in the range of 9–10). Consequently, extra base should minimize protonation of the amines on 3, allowing the desired reaction to occur. At 60 °C with a 20-fold excess of base per equivalent of 3, this reaction took 50 min to reach completion compared to 20 h with 2 equiv of base, demonstrating that the amount of base is a key component of the reaction. To further accelerate the reaction, we heated at reflux instead of 60 °C, leading to a total reaction time of 30 min with 85% isolated yield (Scheme 2).

The final step from CBZ-protected 4 to product 1 was accelerated by transfer hydrogenation with ammonium formate and microwave irradiation. Ammonium formate is a more user-friendly reagent than H₂ gas, and similar microwave-assisted transfer hydrogenation reactions have been performed to deprotect CBZ-protected amines.²⁴ With the addition of ammonium formate under microwave irradiation in a sealed vessel, the reaction was complete after 10 min at 80 °C. An excess of ammonium formate was used because some sublimation was observed during the reaction, and heating above 80 °C increased the amount of sublimation that was observed. This third reaction was complete in 10 min, compared to the reported route that requires a reaction time of 1 d at room temperature with H₂ gas.

Due to the decreases in reaction times for all three steps, it is possible to synthesize DO2A-t-Bu ester, 1, from commercially available cyclen in one day. We have performed the reactions leading to 1 starting from up to 5 g of cyclen with comparable yields to smaller scales, and we expect that even larger scale reactions would behave similarly.²⁵ Furthermore, similar yields to DO2A-t-Bu ester were obtained for DO2A-methyl ester and DO2A-ethyl ester. The ability to rapidly synthesize methyl, ethyl, and t-Bu ester variants of DO2A has the potential to greatly aid studies that use these molecules as intermediates.