We have been so unfortunate as to miss him.

参考译文：我们是如此不幸运结果错过了和他见面。

解答如下

so修饰形容词unfortunate表示程度加强，后面as to miss him为不定式引导结果状语。

so+adj.+as to do sth 如此以致于，不定式引导结果状语从句

Would you be so kind as to do me a favor, please?  

你会这么好心地帮我一个忙, 好吗?

Will you be so kind as to do it for me?  

请你替我做这件事好 吗 ?

She is not so foolish as to do that.  

她不至于愚蠢到干出那样的事来.