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I am at my friends, leaving shortly.

Although leaving shortly, I'm at my friends.

Both OK?

This is more idiomatic/preferred?

I am at my friends and am leaving shortly.

Thanks
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Well, since friends' (with the apostrophe) is a casual expression, I presume this is spoken, so I would expect "I'm at my friends'. (But) I'm leaving shortly." Yours is better than the first two (which do not sound good at all, actually) and natural, but formal.
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Hi, thanks.

Why are my two first sentences no good, but the same constructions in blue are OK?

a. I am at my friends, leaving shortly.

a1. I ran home, talking on the phone.

b. Although leaving shortly, I'm at my friends.

b1. Although I am leaving shortly, I'm at my friends.

b11. Although hungry and tired, I stayed for one more hour.
Because the leaving is not coterminous with the being.